The official manhunt for the assassins who burst into the home of Haiti’s president continued on Thursday, but as some ordinary Haitians set out to capture suspects themselves, setting afire vehicles believed to have been used in the attack, the interim prime minister appealed for calm and to refrain from violence.

“I’m asking everyone to go to their homes,” said Claude Joseph, the interim prime minister, at a news conference Thursday afternoon. “The police have the situation under control.”

The goal, Mr. Joseph said, is to maintain security in the country and get justice for the former President Jovenel Moïse and his family.

Haiti’s police chief, Léon Charles, said over the past day the authorities had arrested six suspects. He also said police had recovered three bodies, “visibly foreigners,” as well as five vehicles believed to have been used in the assassination plot.

But several of those vehicles had been burned by citizens, he said, making it impossible for the police to gather evidence.

“We can’t have vigilante justice,” Mr. Charles said. “Let us do our work. Help us do our work.”

But passions were high on the streets of Pétionville, an affluent suburb of the capital close to where the president lived. A large crowd of people gathered in front of the police station there, demanding to hear from the police chief about the assassins — some of whom were believed to be inside.

Some demanded street justice.

“Burn them,” they cried.

Later, drifting away from the police station, some took their anger into nearby streets, at one point attacking a car dealership. Two protesters were arrested by the police.

A video shared widely on social media shows a crowd of more than 30 Haitians pulling light-skinned men through the footpaths of a dense neighborhood. One of the men was shirtless and had his arms tied with a rope behind his back. The people in the crowd, who appeared to be unarmed, brought the men to the police station, sources told The New York Times.

The police were also surrounding two buildings in which suspects in the assassination had holed up, Helen La Lime, the top U.N. official in Haiti, said at a news conference.

One of the suspects arrested in Haiti is an American citizen of Haitian descent from South Florida, said Haiti’s minister of elections, Mathias Pierre.

On Thursday, just a day after declaring a “state of siege” and a curfew, Mr. Joseph, the interim prime minister, asked people to return to work and said he planned to reopen the country’s main airport.

Reporting contributed from Andre Paulte and Harold Isaac in Haiti.

An already turbulent political landscape in Haiti threatened to descend into further turmoil on Thursday as a power struggle between two competing prime ministers stoked tensions after the assassination of President Jovenel Moïse.

In the hours after the killing, the country’s interim prime minister, Claude Joseph, said he was in charge, taking command of the police and army in what he cast as an effort to ensure order and stability. Mr. Joseph declared a “state of siege” for 15 days, essentially putting the country under martial law, though constitutional experts were unsure whether he has the legal authority to do so.

It was not even clear whether he was really still prime minister.

Two days before his death, Mr. Moïse appointed a new prime minister, Ariel Henry, a neurosurgeon and politician, who was supposed to take up the role this week.

In an interview with The Nouvelliste, a newspaper, Mr. Henry said that Mr. Joseph was “no longer prime minister” and claimed the right to run the government.

“I am a prime minister with a decree that was passed in my favor,” Mr. Henry said, adding that he had been in the process of forming his government.

Mr. Henry said that he “did not want to add fuel to the fire,” but he criticized Mr. Joseph’s decision to impose a state of siege and called for dialogue to ensure a smooth political transition.

President Moïse himself had faced questions about his legitimacy.

For more than year, he had been ruling by decree. Many, including prominent jurists, contended that his term ended in February. Haiti had been rocked by protests against his rule, and also suffered a surge in gang activity that undercut the legitimacy of the government.

Now, there is a new political struggle, and it threatens to undermine the legitimacy either man would need to effectively lead the police, the army and the country itself.

Secretary of State Antony J. Blinken spoke with Mr. Joseph on Wednesday, the State Department said, offering condolences and offering to assist the country “in support of the Haitian people and democratic governance, peace, and security.” Mr. Joseph, speaking at a news conference on Wednesday evening portrayed the conversation as lasting more than 30 minutes.

“We talked about security, elections and a political accord,” he said.

Adding to the challenges for a government in crisis, Haiti, a parliamentary democracy, has no functioning Parliament. There are currently only 10 sitting senators out of 30; the terms of the other 20 have expired. The entire lower house is no longer sitting, because the representatives terms expired last year. Long-planned elections were scheduled for later this year, but it was unclear when or whether they would take place.

The president of what remains of the Haitian Senate, Joseph Lambert, put out a news release on Thursday morning, saying that the Senate “reassures Haitians and the international community that everything will be managed by the national institutions, political forces and civil society to guarantee the continuity of the state and the republican order.”

A Haitian political analyst, Monique Clesca, said that Mr. Moïse had avoided opportunities to hold national elections, and that when the terms of the country’s mayors expired in January, he had installed his own supporters in those positions.

“The objective was always to be the supreme ruler,” Ms. Clesca said. “Eventually to be able to control the whole political apparatus.”

Haiti has a long history of political instability. The country has been rocked by a series of coups in the 20th and 21st centuries, often backed by Western powers, and has been marked by frequent leadership crises that have driven Haitians into the streets in protest.

While the United States and other nations have long supplied Haiti with much-needed aid and financial assistance, including help in recovering from a devastating earthquake in 2010, Western powers have also exerted an overwhelming influence over the country’s political destiny. The United States occupied the country from 1915 to 1934.

France has had a particularly long and difficult relationship with Haiti. More than two centuries ago, Haitians fought to throw off the yoke of colonial France and to bring an end to one of the world’s most brutal slave colonies.

Jacky Dahomay, a French philosopher who served on a French government-mandated commission on relations with Haiti, faulted France and other international actors for failing to help the country establish “truly democratic institutions.” In an interview, he said that only “the law of the strongest” was working in Haiti at the moment and called for the “an international intervention force to restore order.”

News Analysis

There are failed states. And then there is Haiti.

The Caribbean nation is better described as what one analyst once called an “aid state.” It ekes out an existence with the help of billions of dollars from the international community.

The country’s struggles have long captured the world’s attention, but they have not occurred in a vacuum: Outside nations have played a major role, through the brutal exploitation of the past and years of political interference. But damage has also been done by their efforts to help.

Over the past decade, the international community has pumped $13 billion of aid into the country, afraid to let Haiti fail. But the nation-building that aid was meant to support never came about. Instead, Haiti’s institutions became further hollowed out.

The funds stripped leaders of the incentive to carry out the institutional reforms necessary to rebuild the country. Instead, analysts and Haitian activists say, the leaders learned to bet that in times of crisis — and the country has had many — international governments would open their wallets.

For years, the aid has provided vital services and supplies, but it has also bred corruption and violence, and left political paralysis unchecked.

Some Haitian civil society leaders contend, the United States, a large provider of aid, has propped up strongmen and tied the fate of the nation to them.

“Since 2018, we have been asking for accountability,” Emmanuela Douyon, a Haitian policy expert who gave testimony to the U.S. Congress this year, said in an interview. “We need the international community to stop imposing what they think is correct and instead think about the long term and stability.”

Many Haitians in the diaspora are fearing the worst after the assassination of the country’s president, Jovenel Moïse, an act of violence that many consider a potent symbol of the mayhem experienced in the Caribbean nation in recent months.

Rodney Saint-Éloi, a Haitian-Canadian poet and publisher in Montreal, said the assassination of Mr. Moïse was a blow to democracy in Haiti. “It turns all Haitians into assassins, because he was, like it or not, the president of all Haitians,” he said. “It is the failure of a society and of an elite who helped get us to this point.”

Mr. Moïse, killed in an attack early Wednesday on the outskirts of the capital, Port-au-Prince, had presided over a country buffeted by instability, endemic corruption and gang violence. His refusal to cede power had angered Haitians the world over, and many in the diaspora had put off trips home for the past year as kidnappings and other acts of violence became more commonplace.

Because of its chronic instability, Haiti has a large diaspora, with some of the largest communities based in the United States, Canada, France and the Dominican Republic. About 1.2 million Haitians or people of Haitian origin live in the United States, according to 2018 data from the U.S. Census Bureau. But the figure is thought to be higher because of a sizable number of immigrants who are in the country without documentation.

Frantz André, a leading Haitian rights advocate in Montreal, organized a protest in March in which dozens of Haitians demonstrated against what they called Mr. Moise’s political repression. He described Mr. Moïse as a deeply polarizing figure and said that other Haitians abroad were feeling mixed emotions about the president’s killing.

“I don’t think it would be wise to scream victory at his assassination, because we don’t know what will come after and the situation could be even more precarious,” Mr. André said. “Educated people saw him as a threat to democracy, and others have been protesting against him because they have nothing to eat.”

Mr. André added that a sizable minority had supported Mr. Moïse and saw him as a catalyst for change, because he had promoted the idea of giving Haitians outside the country the right to vote and was pushing to change the Constitution.

The Haitian security forces are engaged in what the authorities described as a sweeping manhunt for suspects in the killing of President Jovenel Moïse. Four people were killed, and two more taken into custody after a shootout late Wednesday.

With security forces still hunting for the killers and investigators combing through the evidence from the scene of his assassination, the body of Haiti’s slain president, Jovenel Moïse, was loaded onto an ambulance on Wednesday, bound for a morgue.

A procession of cars was seen speeding away from the presidential residence, but things apparently did not go as planned: Encountering a highway blocked by tires, and hearing gunfire, observers said, the drivers made a quick turnaround.

They needed another route.

The same could be said for Haiti itself on Thursday, a day after its president was shot by a team of assassins described as “well-trained professionals” who had stormed his home on the outskirts of the capital, Port-au-Prince, and then disappeared into the night.

Now, an interim prime minister whose legitimacy was already under question — a replacement was named before the assassination — has declared himself in charge, and put the country under a Haitian version of martial law.

Parliament is riddled with vacancies and inactive. And a country steeped in violence is poised for things to get worse. Late Wednesday, prolonged gunfire could be heard in Port-au-Prince.

“It’s a very grave situation,” said Georges Michel, a Haitian historian and constitutional expert.

The interim prime minister, Claude Joseph, appealed for calm. “Let’s search for harmony to advance together, so the country doesn’t fall into chaos,” he said in a televised address to the nation.

But the country has learned the hard way over the decades, through earthquake and disease, poverty and political turbulence, that chaos feels always near at hand.

“I don’t know what’s going to happen now,” one man said as neighbors gathered to exchange news. “Everything is possible.”

Andre Paultre contributed reporting.

Haiti has been thwarted by outside interests from its very foundation as a country.

For decades, European powers, and later the United States, refused to recognize it as an independent republic.

The Caribbean nation became the world’s first Black-led republic when it declared its independence from France on New Year’s Day 1804. That day, Saint-Domingue, once France’s richest colony, known as the “Pearl of the Antilles,” became Haiti.

It was a land long coveted for its riches of sugar, coffee and cotton, brought to market by enslaved people. Its declaration of independence meant that, for the first time, a brutally enslaved people had wrenched their freedom from colonial masters. But it came only after decades of bloody war.

In 1825, more than two decades after independence, the king of France, Charles X, sent warships to the capital, Port-au-Prince, and forced Haiti to compensate former French colonists for their lost property.

Haiti, unable to pay the hefty sum, was forced into a debt that it had to shoulder for nearly a century. Throughout the 19th century, a period marked by political and economic instability, the country invested little in its infrastructure or education.

In 1915, U.S. troops invaded after a mob killed the Haitian president.

The United States later justified its occupation as an attempt to restore order and prevent what it said was a looming invasion by French or German forces. But U.S. troops reintroduced forced labor on road-construction projects and were later accused of extrajudicial killings.

The widely unpopular occupation ended in 1934, but U.S. control over Haiti’s finances lasted until 1947.

After a series of midcentury coups, the Duvalier family, father-and-son dictators, reigned over Haiti with brute force until the 1980s. Their regime plunged Haiti deeper into debt, and introduced the so-called Tontons Macoutes, an infamous secret police force that terrorized the country.

In the early 1990s, Jean-Bertrand Aristide, a former Roman Catholic priest, was elected president. He was then ousted twice from power over the next 15 years.

Mr. Aristide preached liberation theology, and threatened the establishment by promising economic reforms. After a first coup, he was restored to power. But he left the presidency for good after a second coup in 2004, which was supported by the United States and France. He was exiled to the Central African Republic and, later, to South Africa.

Haiti, with a population of 11 million, is considered the poorest country in the Western Hemisphere.

In 2010, it suffered a devastating earthquake that claimed the lives of about 300,000 people. The country never really recovered, and it has remained mired in economic underdevelopment and insecurity. A cholera outbreak in 2016, linked to U.N. peacekeepers, killed at least 10,000 Haitians and sickened another 800,000.

Then early Wednesday, Jovenel Moïse, who became president in 2017, was assassinated at his residence.

Not long after Haiti’s president was shot to death by assassins who burst into his home early Wednesday, the country’s interim prime minister announced that he had declared an “état de siège” — a state of siege.

To many people around the world watching with alarm as events unfold in Haiti, the term was unfamiliar, even baffling.

But things grew a little clearer when the interim prime minister, Claude Joseph, published details of the order in the official government journal, Le Moniteur.

Haiti is now basically under martial law. For 15 days, the police and members of the security forces can enter homes, control traffic and take special security measures and “all general measures that permit the arrest of the assassins” of President Jovenel Moïse. It also forbids meetings meant to excite or prepare for disorder.

There is one wrinkle. Or two, really.

Only Parliament has the power to declare a state of siege, said Georges Michel, a Haitian historian and constitutional expert. But Haiti at this moment has no functional Parliament. The terms of the entire lower house expired more than a year ago, and only 10 of Haiti’s 30 Senate seats are currently filled.

“Legally, he can’t do this,” Mr. Michel said. “We are in a state of necessity.”

There are actually a few other wrinkles.

Mr. Joseph’s term as interim prime minister is about to end and, in fact, President Moïse had already appointed a replacement, his sixth since taking office.

“We are in total confusion,” said Jacky Lumarque, rector of Quisqueya Universty, a large private university in Port-au-Prince. “We have two prime ministers. We can’t say which is more legitimate than the other.”

It gets worse.

Haiti also appears to have two Constitutions, and the dueling documents say different things about what to do if a president dies in office.

The 1987 version — published in both national languages, Creole and French — deems that if the presidency is vacant for any reason, the country’s most senior judge should step in.

In 2012, however, the Constitution was amended, and the new one directed that the president should be replaced by a council of ministers, under the guidance of the prime minister. Except if, as was Mr. Moïse’s situation, the president was in the fourth year of office. In that case, Parliament would vote for a provisional president. If, of course, there were a Parliament.

Unfortunately, that Constitution was amended in French, but not in Creole. So as it stands, the country has two Constitutions.

“Things are unclear,” said Mr. Michel, who helped write the 1987 Constitution. “It’s a very grave situation.”

Mr. Lumarque lamented the state of his country.

“This is the first time where we’ve seen that the state is so weak,” he said. “There is no Parliament. A dysfunctional Senate. The head of the Supreme Court just died. Jovenel Moïse was the last legitimate power in the country’s governance.”

The assassination of President Jovenel Moïse of Haiti on Wednesday could complicate efforts to contain the Covid-19 pandemic in the Caribbean nation, which has yet to begin vaccinating its citizens, officials from the World Health Organization warned.

Carissa Etienne, the director of the Pan American Health Organization, which is part of the W.H.O., said her organization had made Haiti a priority in recent weeks as reported cases have surged.

“I am hopeful that the arrival of vaccines in the country can start to turn the tide of the pandemic and bring some relief to the Haitian people during these very difficult times,” Dr. Etienne said. “We continue to stand with them now and will redouble our efforts.”

Haiti did not experience the kind of surge early in the pandemic that many experts feared could devastate the country, the poorest in the Western Hemisphere. But the pandemic has grown worse in recent weeks, with a rise in reported cases that experts say is almost certainly an undercount, considering the country’s limited testing capacity.

Last month, Covid-19 claimed the life of René Sylvestre, the president of Haiti’s Supreme Court — a leading figure who might have helped to establish order in the wake of an assassination that has plunged the country into even deeper political uncertainty.

Dr. Etienne’s organization said in an email that while it was too soon to evaluate the impact of the assassination, “further deterioration of the security situation in Haiti could have a negative impact on the work that has been done to curtail Covid-19 infections,” as well as on vaccination plans.

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The organization said that Haiti was also facing challenges from the start of hurricane season and the recent detection of the Alpha and Gamma virus variants on the island. Though “vaccines are expected to arrive shortly” in Haiti, the organization said it did not have a specific delivery date.

In June, Dr. Etienne urged the global community to do more to help Haiti cope with rising coronavirus cases and deaths. “The situation we’re seeing in Haiti is a cautionary tale in just how quickly things can change with this virus,” she said.

Haiti is an extreme example of the “stark inequities on vaccine access,” Dr. Etienne said. “For every success, there are several countries that have been unable to reach even the most vulnerable in their population.”

Across Latin America and the Caribbean, there are millions of people who “still don’t know when they will have a chance to be immunized,” she said.

She said the inequitable distribution of vaccines posed practical and moral problems.

“If we don’t ensure that countries in the South have the ability to vaccinate as much as countries in the North, this virus will keep circulating in the poorest nations for years to come,” Dr. Etienne said. “Hundreds of millions will remain at risk while the wealthier nations go back to normal. Obviously, this should not happen.”

The top United Nations official in Haiti told reporters Thursday that she considered Claude Joseph, the interim prime minister, the person in charge of the country in the aftermath of President Jovenel Moïse’s assassination.

The assertion by the official, Helen La Lime, the head of the United Nations Integrated Office in Haiti, carries weight concerning the question over who is legally authorized to be running the country of 11 million. A former American diplomat, she spoke remotely via teleconference from Port-au-Prince, the capital, after briefing the United Nations Security Council about the Haitian crisis in a private meeting.

Mr. Joseph was supposed to be replaced this week by Ariel Henry, who had been appointed prime minister by Mr. Moïse two days before his assassination. But hours after the killing of the president early Wednesday, Mr. Joseph assumed leadership of Haiti, taking command of the police and army in what he said was an effort to ensure order and stability.

Mr. Henry’s confirmation as prime minister “did not happen,” Ms. La Lime said, and Mr. Joseph “continues to govern,” under Article 149 of the country’s 1987 Constitution.

At the same time, she said, “there are certainly people on all sides of the issue who have different interpretations of Article 149, and that is why it’s important to have a dialogue.”

Ms. La Lime stressed Mr. Joseph’s contention that he intended to hold elections later this year. The country’s Parliament is not functioning, as many members’ terms expired last year, and Mr. Moïse had come under international criticism for failing to call elections.

Ms. La Lime also said that Haiti’s government had made a “request for transitional security assistance” from the United Nations, which once deployed thousands of peacekeepers in the country but withdrew them a few years ago. “Haiti needs to specify what it’s after,” she said.

Regarding the killers of Mr. Moïse, Ms. La Lime said “all efforts must be made to bring these perpetrators to justice.”