UK Prime Minister Boris Johnson has laid flowers at the scene of the fatal stabbing of MP David Amess, which police believe could be linked to Islamist extremism.
Mr Johnson, accompanied by Labour leader Keir Starmer, Commons Speaker Lindsay Hoyle and Home Secretary Priti Patel, laid a wreath at the front of Belfairs Methodist Church in Leigh-on-Sea, where Mr Amess was killed yesterday.
It came after Scotland Yard said the country's most senior counter-terror officer, Deputy Assistant Commissioner Dean Haydon, has formally declared the incident as terrorism and said early investigations have revealed "a potential motivation linked to Islamist extremism".
Mr Amess, 69, who had been an MP since 1983, was fatally injured while meeting constituents.
A 25-year-old man arrested at the scene on suspicion of murder remains in police custody.
In a statement, Scotland Yard said: "Late on Friday, whilst in police custody, the man was subsequently further detained under section 41 of the Terrorism Act 2000 and he is now being held at a London police station.
"On Saturday, detectives were granted a warrant of further detention at Westminster Magistrates' Court, allowing them to keep the man in custody until 22 October, when the warrant expires.
"As part of the fast-paced investigation, officers have attended three addresses in the London area and conducted searches. One of these searches has concluded and the others are ongoing. A post-mortem examination has taken place today."
Official sources told the PA news agency the man is believed to be a British national with Somali heritage.
The force believe the man acted alone and are not seeking anyone else in connection with the matter at this time, but inquiries into the circumstances of the incident are continuing.
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Home Secretary Priti Patel has asked all police forces to review security arrangements for MPs "with immediate effect" following the attack.
Chief constable of Essex Police Ben-Julian Harrington said Mr Amess was "simply dispensing his duties when his life was horrifically cut short".
The Tory veteran, who was described by Prime Minister Boris Johnson as "one of the kindest, nicest, most gentle people in politics", had been an MP since 1983 and was married with five children.
Ms Patel met police and representatives of the security and intelligence agencies after the stabbing, which took place as Mr Amess held a surgery in his Southend West constituency.
"The Home Secretary has asked all police forces to review security arrangements for MPs with immediate effect and will provide updates in due course," the spokesman for Ms Patel said.
The Daily Telegraph said the review would examine Operation Bridger, a nationwide police protective security operation established in 2016 after several threats to MPs following Parliamentary debates on Syria.
Ms Patel will make a statement to Parliament on the review on Monday, The Times reported.
The attack on Mr Amess came just five-and-a-half years after Labour MP Jo Cox was killed by a far right extremist in her Batley and Spen constituency in West Yorkshire.
The Commons Speaker Lindsay Hoyle said police were contacting all MPs to check on their security in the wake of David's killing.
"It is about doing the right things working with the police constabularies right across the United Kingdom because it is about joining that up," Mr Hoyle told BBC2's Newsnight.
"I know that they are contacting all the MPs to check about their safety, to reassure them, because in the end we have got to make sure that is a priority."
He added: "Those people who do not share our values or share democracy, they will not win and we won't let them win. We will continue to look at security, that is ongoing and it will continue."
Mr Hoyle said earlier that while it was right that security was reviewed following the latest incident, it was important to avoid "knee-jerk" reactions.
He told BBC Radio 4's PM programme: "What we want to do is make sure MPs can carry out their duties. We have got to make sure MPs are safe."
His sentiments were echoed by the the Father of the House - the longest-serving sitting MP - Peter Bottomley.
"I predict all over the country this weekend, next weekend and in the months to come, MPs will hold advice sessions. That is what we do," he said.
"There is no perfect security for anybody. My view has always been that in many other walks of life you are at far greater risk than a Member of Parliament.
"MPs may get exceptional publicity. We are not exceptional people. We're ordinary people trying to do an ordinary job as well as we can. We accept the risks.
"The question is - should MPs stop meeting their constituents face-to-face? The answer is we will go on meeting our constituents face-to-face."
It is unclear at this stage whether the attacker had booked an appointment to see Mr Amess at his surgery, was waiting for him or arrived during the surgery.