LONDON — Prime Minister Boris Johnson and other British leaders paid their respects on Saturday morning at a church east of London where a Conservative lawmaker was fatally stabbed a day earlier, as the country grappled with another apparent episode of lone-wolf terrorism.
A somber Mr. Johnson — joined by the opposition leader, Keir Starmer, and other officials — laid flowers outside the Methodist church in Leigh-on-Sea, a sleepy seaside community that was convulsed on Friday when the lawmaker, David Amess, was assaulted during a routine meeting with constituents.
The police arrested a 25-year-old man at the scene and said they were conducting searches at two locations in the London area. The Metropolitan Police formally declared the attack a terrorist episode, with a potential link to Islamist extremism, but they have not yet identified the man, who they said they believed acted alone. The BBC, citing government sources, reported that he was a British national who appeared to be of Somali heritage.
The brutal attack, at midday and in full view of the public, has stunned the British political establishment and fanned questions about the security of members of Parliament. Lawmakers regularly meet their constituents, unprotected, to hear their concerns and grievances in sessions — known as surgeries — that can at times become heated.
It has also rekindled memories of other attacks by radicalized individuals, most recently in February 2020, when a 20-year-old man with a history of extremism was shot and killed by the police after stabbing two pedestrians in South London.
That man, Sudesh Amman, had just been released halfway into a three-year sentence on charges of distributing extremist material and possessing material that could be useful for preparing a terrorist attack. He was being tailed by undercover police, who interrupted the daylight assault on a busy street.
In November 2019, the police shot and killed Usman Khan, 28, on London Bridge after he set off on a frenzied stabbing spree, killing two people and wounding three. Mr. Khan, the British-born son of Pakistani immigrants, had earlier been convicted of being part of a group that plotted to bomb London’s stock exchange.
In April, the Johnson government tightened terrorism laws, mandating that those convicted of serious acts of terrorism serve a minimum of 14 years in prison, under stricter supervision. Some legal critics argue that prolonging prison terms only serve to radicalize offenders even further.
As Scotland Yard scrambled for answers on Saturday, public officials paid tribute to Mr. Amess’s long record of government service.
Commissioner Roger Hirst of the Essex Police, which has jurisdiction over Leigh-on-Sea, said in a statement that it was “a somber moment of reflection to remember a man who worked so hard for his community, who served those he represented passionately and made a real difference for Southend.”
“As we try and come to terms with these tragic events, it is important we remember the man he was and contribution he made,” Commissioner Hirst said.
On the town’s normally tranquil streets, the sudden spasm of violence had not yet fully sunk in. On Saturday morning, the police canvassed residents near the church, seeking witnesses. Chaplains consoled a steady stream of people who visited the area where Mr. Amess was killed.
Alan Dear, 76, a local councilor, spoke tearfully of the lawmaker, who he said had helped him in his own campaign for local office.
“He was just a fantastic person, very kind, loving, gentle man,” Mr. Dear said. “He spent his whole life — 40 years looking after people. All he really wanted was to solve people’s problems.”
More than just an attack on a friend, Mr. Dear said the stabbing had struck at one of the pillars of political life in Britain.
“It was an attack on David, but it was also an attack on democracy in this country,” he said. “It’s very important that we keep in contact with our constituents.”
Mr. Dear said lawmakers should be offered better protections, but not at the cost of those connections with voters. Either way, the attack kicked off an urgent debate over whether current measures are inadequate.
One Conservative lawmaker, Tobias Ellwood, called for face-to-face meetings to be suspended temporarily until a review of security was complete. Another, Michael Fabricant, said it would be safer for members of Parliament to meet constituents by appointment “rather than publicizing in advance a venue and its location with anyone being able to walk in off the street.”
Harriet Harman, a long-serving Labour member of Parliament, told the BBC that she would urge Mr. Johnson to support a special cross-party inquiry to investigate ways to improve security for lawmakers.
Stuart Andrew, the deputy chief whip in the House of Commons, said that while the events of the past day had made him feel “anxious, naturally,” he was determined not to let that deter him and would hold his open constituency meeting on Saturday in honor of Mr. Amess.
The home secretary, Priti Patel, asked the police to review security and to contact each lawmaker. Speaking near the scene of the attack, Ms. Patel said that “we cannot be cowed by any individual or any motivation or people with motives to stop us from functioning to serve our elected democracy.”
Friends of Mr. Amess said he was known for his passionate campaigning on behalf of animal rights, as well as for his social conservatism. He supported a ban on fox hunting, a position that put him at odds with some fellow Conservatives, and sponsored legislation outlawing the cruel tethering of horses.
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 attack, said 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.