Sir David Amess has died hours after being stabbed multiple times on Friday afternoon.
The Conservative MP, who represented Southend West in Essex, was reportedly attacked by a man who walked into a church in Leigh-on-Sea where he was hosting a constituency surgery.
The 69-year-old remained in the Belfairs Methodist Church for at least two hours following the incident at around 12.05pm receiving treatment, according to reports.
An air ambulance was at the scene, as were other emergency vehicles, with the area sealed off and placed on lockdown.
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Essex Police arrested a man, then later confirmed the victim had died.
The force tweeted just after 3pm: "A man has been arrested on suspicion of murder after a man was stabbed in #LeighonSea. We were called to Eastwood Road North shortly after 12.05pm.
"Sadly, a man later died. A man was arrested at the scene. We are not looking for anyone else."
Counter-terrorism police are understood to be assisting the investigation.
It is not clear if the MP's wife, Julia Arnold, was with him at the time, although she does help him with casework.
Local councillor John Lamb, who was at the surgery with Mr Amess, described the MP as an "amicable" family man.
"He's a family man, he's got four daughters and a son," he told the PA news agency.
A Conservative MP has been stabbed multiple times in an incident at his constituency surgery.
A Conservative MP has been stabbed multiple times in an incident at his constituency surgery.
"He's always trying to help people and especially refugees he's tried to help.
"He's a very amicable person and he does stick by his guns, he says what he believes and he sticks by it."
Friend Tim Gocher said: “I’m absolutely gutted. He’s a very good friend of mine. Everyone knows him. He’s done this surgery for years. Everyone knows him, knows he is dedicated to helping everyone.”
Mr Amess had tweeted earlier this week that the surgery would be going ahead.
The incident has caused shock waves to spread through the usually quiet seaside neighbourhood.
Witnesses on the scene recalled the chaos that unfolded after the attack.
Anthony Gavin Fitch, 38, said he was just getting out of his car when he saw a woman wearing a Conservative Party badge phoning the police outside the church.
Police officers and ambulance crew attend following the stabbing of UK Conservative MP Sir David Amess
"She was saying 'get here as soon as you can, he's still in the building'."
Referring to the man who was arrested, Mr Fitch added: "He was taken away by two officers with armed police behind him. He didn't make any noise."
He went on to say a woman inside the church told "there was an altercation" and Mr Amess was stabbed three times.
Lee Jordison, 40, who also witnessed the aftermath, was told that a woman came out from the church screaming "he's not breathing".
Speaking to The Mirror, he said: "I was working in our shop about 100 metres away when I was alerted to ambulances and police cars going past.
"I took a walk down the road and saw police setting up a cordon. I spoke to someone walking up the road and they said there was a stabbing in the church.
"There were lots of ambulances and police with machine guns.
"I heard from a couple of people that a lady came out screaming phoning the police saying 'please get here quickly he's not breathing."
Brendan Cox, widower of the late Labour MP Jo Cox, who was murdered in an attack in 2016, said the stabbing was "as cowardly as it gets".
He tweeted: "Attacking our elected representatives is an attack on democracy itself. There is no excuse, no justification. It is as cowardly as it gets."
Sir David Amess has been stabbed multiple times in an incident in his constituency
Media opportunities to film the start of a Cabinet meeting on Friday have been cancelled, as was a planned interview with Prime Minister Boris Johnson, which was due to take place afterwards.
Mr Amess had been an MP since 1983, first for Basildon and since 1997 for Southend West.
He was a well-known backbencher who never held a ministerial or shadow ministerial post during his 38 years in parliament, instead of campaigning on issues including animal rights.
He was considered to be on the right of the Tory party and opposed bills furthering LGBT rights, including equal age of consent and same-sex marriage.
Earlier this year he spoke out about anti-social behaviour and knife crime and was a firm supporter of Brexit.
He was created a Knight Bachelor in the 2015 New Year Honours for political and public service.
He is a member of the Imperial Society of Knights Bachelor.