“The early investigation has revealed a potential motivation linked to Islamist extremism,” according to a statement from Metropolitan Police at New Scotland Yard.
Authorities said they believed the suspect acted alone and they are “not seeking anyone else in connection with the incident at this time. However, enquiries into the circumstances continue,” the statement added.
Conservative Party politician Sir David Amess, 69, died after being stabbed during a meeting with constituents overnight.
A 25-year-old man has been arrested on suspicion of murder they added.
“We were called to an address in Eastwood Road North shortly after 12.05pm today (Friday 15 October) [22.05 AEDT],” police said in a statement.
“We attended and found a man injured. He was treated by emergency services but, sadly, died at the scene.
“A 25 year-old man was quickly arrested after officers arrived at the scene on suspicion of murder and a knife was recovered.
“He is currently in custody. We are not looking for anyone else in connection with this incident.”
The lawmaker was attacked during a regular constituency meeting at Belfairs Methodist Church in a residential area of Leigh-on-Sea, a seaside town 62 kilometres east of London.
Aerial footage showed several ambulances and an air ambulance waiting nearby to the church.
John Lamb, a local councillor, said Amess had not been taken to hospital more than two hours after the attack and the situation was “extremely serious.”
“He was stabbed several times,” the Southend councillor told Reuters: “We’re not sure how serious it is but it’s not looking good.”
Amess has been a member of Parliament for Southend West, which includes Leigh-on-Sea, since 1997, but has been a lawmaker since 1983.
The 69-year-old is survived by his wife Julia Arnold and their five children — one son and four daughters, one of whom is British actress Katie Amess.
Prime Minister’s Boris Johnson is yet to comment on the incident but wife, Carrie Johnson shared a tribute on social media.
“Absolutely devastating news about Sir David Amess,” Mrs Johnson tweeted.
“He was hugely kind and good. An enormous animal lover and a true gent. This is so completely unjust. Thoughts are with his wife and their children.”
Scotland’s first minister Nicola Sturgeon said: “Elected representatives from across the political spectrum will be united in sadness and shock today.
“In a democracy, politicians must be accessible and open to scrutiny, but no-one deserves to have their life taken while working for and representing their constituents.”
While Conservative MP Tracey Crouch said “a little light went out in Parliament today”.
Violence against British politicians is rare, but in June 2016 Labour Party lawmaker Jo Cox was fatally stabbed and shot in her northern England constituency. A far-right extremist was convicted of her murder.
Cox’s widower, Brendan Cox, tweeted: “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.”
A charity set up in memory of the MP said it was “horrified” by reports of the stabbing.
“We are thinking of him, his family and loved ones at this distressing time,” The Jo Cox Foundation said in a tweet.
Politicians from across the political spectrum expressed shock shortly after news of the stabbing first broke.
Labour Party leader Keir Starmer tweeted that it was “Horrific and deeply shocking news. Thinking of David, his family and his staff.”
Former Prime Minister David Cameron, a Conservative, tweeted: “Very alarming and worrying news reports coming from Leigh-on-Sea. My thoughts and prayers are with Sir David Amess and his family.”
Fellow Conservative MP Iain Duncan Smith wrote: “My thoughts are with David Amess MP and his family at this awful time. Praying for a full recovery following this appalling, shocking news. This angry, violent behaviour cannot be tolerated in politics or any other walk of life.”
British lawmakers are protected by armed police when they are inside Parliament, but have no such protection in their constituencies.
Two other British lawmakers have been attacked this century during their “surgeries,” regular meetings where constituents can present concerns and complaints.
Labour legislator Stephen Timms was stabbed in the stomach and injured in May 2010 by a female student radicalised by online sermons from an al-Qaida-linked preacher.
In 2000, Liberal Democrat lawmaker Nigel Jones and his aide Andrew Pennington were attacked by a man wielding a sword during such a meeting. Pennington was killed and Jones injured in the attack in Cheltenham, England.