Mr. Amess was also a vocal supporter of the Iranian opposition group Mujahedeen Khalq, or M.E.K., which campaigns for the overthrow of Iran’s government. The group has attracted a bipartisan list of American backers, including John R. Bolton, who served as a national security adviser to President Donald J. Trump, and Howard Dean, a onetime chairman of the Democratic Party.
There was no evidence linking the attack to Mr. Amess’s support for the M.E.K. Though the group was once designated as a terrorist organization by the United States, Britain and the European Union, all three removed that designation several years ago.
David Jones, a Conservative member of Parliament and a leader of the British Committee for Iran Freedom, which backs the M.E.K., hailed Mr. Amess as “a champion of human rights and democracy in Iran for more than three decades.”
For residents of Leigh-on-Sea, the senselessness of the attack was difficult to comprehend, let alone accept.
“I just want to know, why?” said Audrey Martin, 66, who was buying groceries as Mr. Johnson and the other leaders arrived to lay flowers. “Why has he done it and why has he chosen to come to Leigh-on-Sea?”
Fidelia McGhee, 48, who lives near the site of the attack, said that Mr. Amess had always championed local causes. While she described herself as a longtime Labour voter, she praised him as a kind, committed politician. She called the attack “the stuff of nightmares” that would leave an indelible mark on the town.
“It is quite tragic,” she said. “I think we’ve lost something we will never get back.”
Mark Landler and Stephen Castle reported from London, and Megan Specia from Leigh-on-Sea, England.