HONG KONG — Jimmy Lai, a pro-democracy media figure, and several of Hong Kong’s most prominent opposition campaigners were sentenced on Friday to prison terms of eight months to 18 months for holding an unauthorized peaceful protest.

Supporters of the defendants say the prosecutions are the latest sign of the fundamental transformation that Beijing has sought to impose on Hong Kong. Until recently, the city had long been a bastion of free speech. Now, the sentences send an unmistakable message that activism carries severe risks for even the most internationally recognized opposition figures.

The court sentenced Mr. Lai, 73, a media tycoon who founded Apple Daily, an aggressively pro-democracy newspaper, to 12 months in prison. Martin Lee, an 82-year-old lawyer, often called Hong Kong’s “father of democracy,” was handed a suspended 11-month prison term, meaning he would avoid being put behind bars if he is not convicted of another crime in the next two years.

overhauled Hong Kong’s electoral system to cement the pro-Beijing establishment’s grip on power. Protests have been largely barred during the pandemic, and self-censorship in the media and arts, which are under intense official pressure, is a growing concern.

Over a period of months in 2019, hundreds of thousands of people joined antigovernment demonstrations in one of the greatest challenges to the Communist Party in decades. The sentences imposed on Friday, added to the measures already taken against dissent, are likely to chill participation in such protests in the future.

“It’s very clear that the approach has changed radically, not just by courts and police,” said Sharron Fast, a media law lecturer at the University of Hong Kong. “The emphasis is on deterrence; the emphasis is on punishment. And with large-scale assemblies, the risk is very high.”

a march on Aug. 18, 2019, that followed a gathering in Victoria Park on Hong Kong Island. The rally in the park had been permitted by the police, but the authorities, citing the violence at earlier protests, had not approved plans for demonstrators to march about two miles to government headquarters afterward.

He has traveled the world, including many trips to Washington, to lobby for that cause. Such internationally focused activism is now banned under the national security law.

Mr. Lai, the media mogul, was smuggled into Hong Kong from mainland China as a child and worked his way up from factory laborer to clothing company tycoon. He then put his wealth into crusading, tabloid-style publications that have been sharply critical of the authorities in Beijing and Hong Kong.

Mr. Lai also faces a fraud case and charges of collusion with a foreign country under the security law for allegedly calling for sanctions against Hong Kong. In a separate hearing on Friday, prosecutors added two more national security charges, accusing Mr. Lai of conspiracy to commit subversion and obstructing justice.

In the illegal assembly case, the court rejected defense arguments that the procession after the rally was necessary to help protesters safely clear out of the crowded park, or that potential imprisonment for a nonviolent march would infringe on the rights to free speech and assembly that have traditionally been protected in Hong Kong.