Essex Police said a suspect has been arrested on suspicion of murder, as tributes poured in to the “hugely kind and good” MP, who was one of the longest-serving in parliament.
The prime minister called him “one of the kindest, nicest, most gentle people in politics” and praised “almost 40 years of continuous service to the people of Essex and the whole of the United Kingdom”.
Counter-terror police are involved in the early investigation into the stabbing and are working to establish the potential motivation.
There has not yet been any confirmation of whether the incident is being considered terror-related.
The father-of-five is the second sitting MP to be killed in five years, following the attack on Labour MP Jo Cox in 2016.
She was murdered by a neo-Nazi outside a constituency surgery in Birstall, West Yorkshire.
Her widower, Brendan Cox, wrote on Twitter: “My thoughts and love are with David’s family. They are all that matter now. This brings everything back.
“The pain, the loss, but also how much love the public gave us following the loss of Jo. I hope we can do the same for David now.”
Mr Cox said attacking MPs was an “attack on democracy itself”, adding: “There is no excuse, no justification. It is as cowardly as it gets.”
Police were called to the scene shortly after noon on Friday and said the surrounding roads have been closed off.
A spokesperson for Essex Police said: “We were called to an address in Eastwood Road North shortly after 12.05pm on Friday to reports Sir David, 69, had been stabbed.
“Despite the best efforts of paramedics, sadly, he died at the scene. A 25 year-old man was arrested on suspicion of murder and is currently in custody.
“We have recovered a knife from the scene and are not looking for anyone else in connection with the incident.”
Investigators were appealing for anyone who saw anything or has CCTV, dash cam or doorbell footage to come forward.
One witness called the incident “very distressing” and described seeing someone being taken out of the church and put into the back of a police car, before he was asked to leave the area.
“At that point obviously we knew something serious had happened,” he told LBC radio.
“Within five minutes we'd been informed by someone who was actually in the building what had happened and apparently he (Sir David ) had been stabbed quite a few times. It's very distressing.”
Sir David had advertised the surgery, which is a regular opportunity for constituents to air their views and seek assistance, on his Twitter page on Tuesday, giving the location and contact details to book an appointment.
In a book published last year, Ayes & Ears: A Survivor's Guide to Westminster, Sir David reflected on the impact of the murder of Ms Cox and other MPs.
“We all make ourselves readily available to our constituents and are often dealing with members of the public who have mental health problems, it could happen to any of us,” Sir David wrote.
He said that heightened security concerns had caused MPs to change how they interact with the public, and affected the “great British tradition of people openly meeting their elected politicians” at constituency surgeries.
Sir David said he had experienced “nuisance” from people turning up at his home in the past, and received frequent abuse on social media.
He called for the law to be changed so that abusive commenters could be identified, saying that he was frustrated that “these ignorant cowards are allowed to get away with appalling behaviour”.
Tributes poured in from across the political spectrum on Friday for Sir David, who was one of parliament’s longest-serving MPs.
Priti Patel, the home secretary, said herved the people of Southend with endless passion, energy and integrity”.
“It represents a senseless attack on democracy itself,” she added. “Questions are rightly being asked about the safety of our country's elected representatives and I will provide updates in due course.
Former prime minister David Cameron wrote on Twitter: “David Amess was a kind and thoroughly decent man - and he was the most committed MP you could ever hope to meet. Words cannot adequately express the horror of what has happened today.”
The Labour leader Sir Keir Starmer called it a “dark and shocking day” and praised Sir David’s public service.
“The whole country will feel it acutely, perhaps the more so because we have, heartbreakingly, been here before,” he added.
“Let us come together in response to these horrendous events. 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.”
Sadiq Khan, the mayor of London, said he was “deeply saddened” and added: “He loved being an MP and was a great public servant. It is just awful. My thoughts and prayers, and those of all Londoners, are with David's loved ones at this time of unimaginable grief.”
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”.
Sir David was first elected to parliament to represent Basildon in 1983, and then stood for election in Southend West in 1997.
He was the chair of the All-Party Parliamentary British-Qatar Group, and recently met the country's emir in Doha.
On his website, he listed his main interests as “animal welfare and pro-life issues”. Sir David is survived by his wife and five children.
Essex Police said anyone with information should contact the force quoting incident 445 of 15 October.
Additional reporting by PA
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