LEIGH-ON-SEA, England — For the second time in little more than five years, a British lawmaker meeting with constituents was killed in full view of the public, this time in a genteel seaside town, where the victim, a Conservative Party member of Parliament, was fatally stabbed on Friday inside a church.
The attack, which the authorities declared a terrorist attack early Saturday, stunned Britain’s political establishment, raising questions about the security of lawmakers at a time when the country is already on edge, unnerved by shortages of food and fuel, and frayed by a political culture that has become increasingly raw and combative in the aftermath of Brexit.
“The early investigation has revealed a potential motivation linked to Islamist extremism,” the police said.
The lawmaker, David Amess, 69, was a long-serving member of the House of Commons known for his soft-spoken manner and hard-line views on Brexit. He was engaged in the everyday political routine of meeting with constituents when the attack occurred in Leigh-on-Sea, on the mouth of the Thames, about 40 miles east of London.
The police said they had arrested a 25-year-old man on suspicion of murder and had recovered a knife at the scene. But they did not identify the assailant.
Ben-Julian Harrington, the chief constable of the Essex Police, called it a “tragic day,” in which the life of a public servant was “horrifically cut short.”
The death of Mr. Amess, known as much for his campaigning on behalf of animal welfare as for his criticism of the European Union, evoked a similar attack in 2016, days before Britons voted in a referendum to leave the European Union. Jo Cox, a Labour lawmaker who opposed Brexit, was killed when a right-wing extremist targeted her outside a meeting with constituents.
In 2010, another Labour lawmaker, Stephen Timms, was stabbed twice in the abdomen by an Islamist extremist, but survived.
Mr. Amess’s death shocked members of Parliament, who expressed outrage over the attack, paid tribute to his long government service and recoiled at another example of sudden violence inflicted on a politician making the rounds with the local constituency.
“David was a man who believed passionately in this country and its future and we’ve lost today a fine public servant and a much loved friend and colleague,” a visibly shaken Prime Minister Boris Johnson said in a televised statement.
Mr. Johnson described Mr. Amess as “one of the kindest, nicest, most gentle people in politics,” who, he added, had an “outstanding record in passing laws to help the most vulnerable.”
The speaker of the House of Commons, Lindsay Hoyle, said in a statement, “This is an incident that will send shock waves across the parliamentary community and the whole country.” Mr. Amess, he said, had “built a reputation for kindness and generosity” over almost four decades in government.
In Britain, most members of Parliament hold regular meetings, known as surgeries, to allow constituents to raise issues of concern. While the gatherings allow politicians to maintain contact with voters, they can also make lawmakers, who often travel without protection, vulnerable to security breaches. During the 2019 general election, lawmakers complained about being targeted by abuse on social media, which some feared would mutate into violent attacks on the street.
Mr. Amess had been scheduled to hold a meeting with voters at the Belfairs Methodist Church in Leigh-on-Sea, a district of Southend, when the attack occurred. Photographs taken at the scene showed a number of emergency responders and a cordoned-off area around the church. The police said that officers had responded to reports of a stabbing shortly after 12:05 p.m., and that Mr. Amess had died at the scene.
“We are not looking for anyone else in connection with the incident and do not believe there is an ongoing threat to the wider public,” the police said.
A father of five, Mr. Amess first entered Parliament in 1983, when Margaret Thatcher led the Conservative Party. He initially represented the seat of Basildon in Essex, where his election consolidated a groundswell of support for the Conservatives in that area. He switched constituencies to Southend West in 1997, a seat that he held in every subsequent general election.
A Roman Catholic who campaigned against abortion, Mr. Amess was a social conservative and a staunch supporter of the British monarchy.
Members of the community gathered in a Catholic church on Friday evening where a Mass was said for Mr. Amess. By then, a small memorial for Mr. Amess had taken shape on the street that passes the church where he was attacked.
“This was one particular lunatic who decided to take extreme action,” said Alan Hart, a local councilor, who had consulted with Mr. Amess a number of times and described him as a “brilliant” representative, even if the two did not always agree on politics.
Mr. Hart said that while the attack was worrying, politicians must be able to hold intimate, face-to-face gatherings with voters in the communities they represent. “We have a very healthy political scene in this country,” he said. “It’s important that this accessibility continues.”
Fears about the vulnerability of lawmakers spiked after the attack on Ms. Cox, who was shot and stabbed by a right-wing extremist at a meeting in her parliamentary constituency in West Yorkshire, in northern England. That attack took place in the fevered days before the Brexit referendum, and the assailant, Thomas Mair, an unemployed gardener, was sentenced to life in prison.
Ms. Cox’s husband, Brendan Cox, reacted to the news of the latest attack on Friday on Twitter. “Attacking our elected representatives is an attack on democracy itself,” he wrote. “There is no excuse, no justification. It is as cowardly as it gets.”
Across the political spectrum, lawmakers and other prominent Britons recalled Mr. Amess’s gentle manner and work on behalf of animals.
“He was hugely kind and good,” said Carrie Johnson, the wife of the prime minister, on Twitter. “An enormous animal lover and a true gent. This is so completely unjust. Thoughts are with his wife and their children.”
“Heartbroken,” wrote Tracey Crouch, a fellow Conservative lawmaker. “I could write reams on how Sir David was one of the kindest, most compassionate, well liked colleagues in Parliament. But I can’t. I feel sick. I am lost. Rest in Peace. A little light went out in Parliament today. We will miss you.”
In Leigh-on-Sea, known for its annual regatta and folk festival, news of the attack reverberated through normally tranquil tree-lined streets.
“This doesn’t really happen, this is a nice quiet area,” said Alysha Codabaccus, 24, who lives in an apartment a few doors down from the church. “I mean, it literally happened in a church.”
At Mojo’s Seafood, a small white shack that serves fresh fish from the nearby coastline, the customers expressed horror and sadness. One remarked on the impact on Mr. Amess’s family. “He’s got five kids,” the man said quietly.
Lee Jordison, who works at a butcher shop 100 yards from the church, said he had heard sirens and seen armed officers running up the street, shattering the typical autumn afternoon quiet, and had known instantly that something was very wrong. He said a shaken woman had told him that people ran from the church screaming, “Please get here quick, he’s not breathing!”
Mr. Jordison said he had met Mr. Amess a few times. “He always used to visit our shop,” he said. “He was a very nice guy from the time I met him. He had a lot of time for the community.”
Megan Specia reported from Leigh-on-Sea, and Stephen Castle and Mark Landler from London.