Conservative MP Sir David Amess has died after being stabbed 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 said they recovered a knife and were not looking for anyone else in connection with the incident.
Prime Minister Boris Johnson said “our hearts are full of shock and sadness” at the loss of “one of the kindest” people in politics.
Sir David, 69, had been an MP since 1983 and was married with five children.
Mr Johnson 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,” he said.
Health Secretary Sajid Javid said he was “a great man, a great friend, and a great MP, killed while fulfilling his democratic role”.
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 said they received reports of a stabbing shortly after 12:05 BST and found a man injured.
He was treated by emergency services but died at the scene.
Sir David is the second 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.
Who Was Sir David Amess?
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.
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.
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.”
Another priest, Father Kevin Hale, said there was a “great sense of incredulity” about the attack.