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.
Sir David, 69, had been an MP since 1983 and was married with five children.
Health Secretary Sajid Javid said he was “a great man, a great friend, and a great MP, killed while fulfilling his democratic role”.
Chancellor Rishi Sunak said: “The worst aspect of violence is its inhumanity. It steals joy from the world and can take from us that which we love the most.
“Today it took a father, a husband, and a respected colleague. All my thoughts and prayers are with Sir David’s loved ones.”
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 sitting MP to be killed in the last 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.”
MPs United In Shock & Sadness
Political differences have been set aside; MPs are united in expressing their shock, sadness and affection for a colleague and a friend.
Many of them will themselves have spent the day in church halls, supermarkets and high streets doing just what Sir David Amess had been: meeting their constituents face-to-face.
When you talk to MPs, they’ll often tell you that it’s a fundamental part of the job and one they really value.
The death of Sir David Amess is both a personal tragedy – and a terrible reminder of the very real risks they all face.
Speaker of the House of Commons Sir Lindsay Hoyle said: “This is an incident that will send shockwaves across the parliamentary community and the whole country.”
He said he was shocked and deeply distressed by the killing of a “lovely man”, and said in the coming days they would need to examine MPs’ security.
Labour London Mayor Sadiq Khan said he was “deeply, deeply saddened” and described Sir David as “a great public servant”.
Liberal Democrat leader Sir Ed Davey said it was “a truly terrible day for British politics but most importantly of all our prayers are with all the people who loved David”.
Flags around Parliament are being flown at half-mast.
What Security Do MPs Have?
• When they are in Parliament, MPs are well protected by a specialist armed police department called Parliamentary and Diplomatic Protection
• For most MPs there is not the same close protection when they are in their constituencies
• Parliament offers MPs and their staff guidance on security, including tips on how to run a safe constituency surgery, suspicious post and home security
• Parliament will pay for MPs to have some security measures installed – such as security alarms and shutters
• After the murder of Jo Cox in 2016, the spending on MPs’ security rose from £170,576 in 2015/16, to £4.5m two years later