Conservative MP Sir David Amess has died after being stabbed multiple times at his constituency surgery in Essex.
Police said a 25-year-old man was arrested on suspicion of murder after the attack at a church in Leigh-on-Sea.
They recovered a knife and are not looking for anyone else in connection with the incident. A counter terrorism team will lead the investigation.
Boris Johnson has spoken of his shock and sadness at the loss of “one of the kindest” people in politics.
Home Secretary Priti Patel has asked all police forces to review security arrangements for MPs “with immediate effect”, a Home Office spokesman said.
Sir David, 69, had been an MP since 1983 and was married with five children. He is the second serving MP to be killed in the past five years, following the murder of Labour MP Jo Cox in 2016.
She was killed outside a library in Birstall, West Yorkshire, where she was due to hold a constituency surgery.
Town in grief: ‘David Amess was my best friend’
The investigation into the attack will be led by the Metropolitan Police’s specialist Counter Terrorism Command, who will determine whether it was a terrorist incident.
A government source told the BBC the man arrested is a British national who, according to initial inquiries, is of Somali heritage.
The prime minister said Sir David had an “outstanding record of passing laws to help the most vulnerable”.
“David was a man who believed passionately in this country and in its future. We’ve lost today a fine public servant and a much-loved friend and colleague,” said Mr Johnson.
Boris Johnson said Sir David Amess was one of the “kindest, nicest, most gentle people in politics”
Ms Patel said the killing “represents a senseless attack on democracy itself”, adding that “questions are rightly being asked about the safety of our country’s elected representatives”.
Speaker of the House of Commons Sir Lindsay Hoyle, who spoke to the home secretary on Friday, also said MPs’ security would have to be examined. He said Sir David’s death would “send shockwaves across the parliamentary community and the whole country”.
Sir David, who represented Southend West, was holding a constituency surgery – where voters can meet their MP and discuss concerns – at Belfairs Methodist Church in Eastwood Road North.
Essex Police Chief Constable Ben-Julian Harrington said they received reports of a stabbing shortly after 12:05 BST and within minutes, officers found Sir David with multiple injuries.
Police and paramedics “worked extremely hard” to save him but he died at the scene, Mr Harrington said.
Armed police were seen outside the church where Sir David met consituents
Map showing the location of the killing of Sir David Amess
Police have appealed for anyone who saw the attack, or who has footage from CCTV, dash cams or video doorbell to contact them.
Southend councillor John Lamb told the BBC that Sir David moved his surgeries to different locations around the constituency “to meet the people” and said the attack was “absolutely dreadful”.
“We’ve lost a very good, hard working constituency MP who worked for everyone,” he said.
A Conservative backbencher for nearly forty years, Sir David entered Parliament in 1983 as the MP for Basildon.
He held the seat in 1992, but switched to nearby Southend West at the 1997 election.
Raised as a Roman Catholic, he was known politically as a social conservative and as a prominent campaigner against abortion and on animal welfare issues.
He was also known for his championing of Southend, including a long-running campaign to win city status for the town.
Father Jeff Woolnough, parish priest at nearby St Peter’s Catholic Church, told the BBC: “Sir David was a great, great man, a good Catholic and a friend to all.
“He’s died doing that, that’s the remarkable thing. He’s died serving the people.”
He led a vigil at the church on Friday evening in memory of Sir David, describing him as “Mr Southend”.
“Have you ever known Sir David Amess without that happy smile on his face?” he asked. “He carried that great east London spirit of having no fear and being able to talk to people and the level they’re at. Not all politicians, I would say, are good at that.”
Labour leader Sir Keir Starmer said it was a “dark and shocking day”, adding that “we have, heartbreakingly, been here before” with the death of Jo Cox.
“We will show once more that violence, intimidation and threats to our democracy will never prevail over the tireless commitment of public servants simply doing their jobs,” he said.