A 25-year-old man is in custody and police have launched a murder probe following the death of the 69-year-old Conservative MP from Essex in southeast England.
Britain's Prime Minister expressed his "shock" and "sadness" on Friday following the death of MP David Amess, killed in a stabbing attack during meetings with constituents.
Sir David Amess, 69, was the MP for the ruling Conservative Party in Southend West, in the southeastern county of Essex.
A 25-year-old man has been arrested on suspicion of murder after the attack, which happened around midday in the English coastal town of Leigh on Sea.
The attack was reported by Essex Police, who later said the stabbing victim had died. They later confirmed that the victim was Amess.
Boris Johnson said he was "full of shock and sadness" at the death of the MP who was "killed in his constituency surgery, in a church, after almost 40 years of continuous service to the people of Essex and the whole of the United Kingdom."
"He was one of the kindest, nicest, most gentle people in politics," he added.
Essex police said they were called to reports of a stabbing in Eastwood Rd North in Leigh-on-Sea at 13h CEST.
Amess had announced that he would be holding his constituency surgery in a church in the same street.
"This was a difficult incident but our officers and paramedics from the East of England Ambulance Service, worked extremely hard to save Sir David. Tragically, he died at the scene," Chief Constable Ben-Julian Harrington said in a statement.
Police arrested a man at the scene and recovered a knife and are not looking for anyone else in relation to the incident.
The Metropolitan Police's Specialist Counter Terror Command to lead the investigation.
"It will be for investigators to determine whether or not this may have been a terrorist incident. As always they will keep an open mind," Chief Constable Ben-Julian Harrington said.
Britain's political class have paid tribute to Amess throughout the day and expressed their dismay at his killing.
The Speaker of the House of Commons, Sir Lindsay Hoyle, said he is "shocked and distressed by the killing of Sir David Amess".
"This is an incident that will send shockwaves across the parliamentary community and the whole country. In the coming days we will need to discuss and examine MPs' security and any measures to be taken but for now, our thoughts, and prayers are with David's family, friends and colleages," he went on to say.
The leader of the main opposition Labour party, Keir Starmer, has described it as "horrific and deeply shocking news".
"Thinking of David, his family and his staff," he added on Twitter.
Other politicians have also offered their support to Amess and his family including Nicola Sturgeon, leader of the Scottish Nationalist party (SNP). She wrote: "Elected representatives from across the political spectrum will be united in sadness and shock today."
"In a democracy, politicians must be accessible and open to scrutiny, but no-one deserves to have their life taken while working for and representing their constituents," she also said.
Sir Ed Davey, leader of the Liberal Democrats party, said it was "a truly terrible day for British politics".
"My thoughts are with David's wife and children, the wider family friends and David's Community," he added.
In 2016, Labour MP Joe Cox was fatally stabbed in her constituency. A far-right extremist was convicted of her murder.
Brendan Cox, the husband of the late Joe Cox, has reacted to the attack on Twitter, writing: "Attacking our elected representatives is an attack on democracy itself. There is no excuse, no justification. It is as cowardly as it gets."
Tracey Crouch, a colleague of Amess on the Conservative benches in the House of Commons, described him as "one of the kindest, most compassionate, well-liked colleagues in Parliament."
Fellow Conservative lawmaker, Andrea Leadsom, said of Amess that he was "one of the loveliest men I've met, and so kind and generous."