British Prime Minister Boris Johnson on Monday vowed that democracy would conquer "acts of evil", as emotional MPs lauded their colleague David Amess after he was stabbed to death while meeting constituents.

Police are investigating whether a suspect arrested at the scene of Friday's attack in a church hall was motivated by Islamist extremism, stoking fears for the safety of elected representatives.

Members of the House of Commons, most dressed in black, observed a minute's silence at the start of a special debate. Many then called for an end to the bitterly divisive rhetoric that has swelled since Britain's 2016 Brexit referendum.

Amess was a well-respected member of parliament for Southend West, in Essex, east of London
Amess was a well-respected member of parliament for Southend West, in Essex, east of London AFP / Adrian DENNIS

They recalled the Conservative Amess' bipartisan cooperation, his ebullient sense of humour, and his deep Catholic faith: one remembered him inadvertently getting a packet of cough sweets blessed by the pope on a visit to the Vatican.

"We will cherish his memory. We will celebrate his legacy," Johnson told the debate, which ended with MPs filing in cross-party procession to a religious remembrance service.

"And we will never allow those who commit acts of evil to triumph over the democracy and the parliament that to Sir David Amess, meant so much," he added.

Conservative MP David Amess, who was killed in a church hall on Friday
Conservative MP David Amess, who was killed in a church hall on Friday AFP / Jonathan WALTER

Home Secretary Priti Patel has ordered a review of security measures for parliamentarians and vowed "to close any gaps" in security provision.

Map of Britain locating attacks against British MPs since 1979.
Map of Britain locating attacks against British MPs since 1979. AFP / Bertille LAGORCE

Amess was one of Britain's longest-serving and most respected MPs. He campaigned on a range of causes, including obtaining city status for the seaside town of Southend, east of London, which he represented.

Queen Elizabeth II has now formally granted the request, Johnson said to cheers.

Earlier, Amess' widow Julia and other members of his family visited the scene of the attack in Leigh-on-Sea, near Southend, wiping away tears as she reviewed a sea of floral tributes.

MPs packed the House of Commons for prayers and a minute's silence in memory of David Amess
MPs packed the House of Commons for prayers and a minute's silence in memory of David Amess PRU / Handout

On Sunday, his family said they were "absolutely broken" by his death and made a plea for tolerance.

"Set aside hatred and work towards togetherness. Whatever one's race, religious or political beliefs, be tolerant and try to understand," they added, after the second killing of a UK politician in five years.

Jo Cox, of the opposition Labour party was killed by a far-right extremist just before the Brexit referendum in 2016.

Cox's constituency in northern England is now represented by her sister, Kim Leadbeater, who recalled at Monday's debate "physically trembling" when she was told about that attack.

Tributes to Amess were left in Parliament Square
Tributes to Amess were left in Parliament Square AFP / Tolga Akmen

"And it breaks my heart to think that another family has had to experience that phone call, and the nightmare which follows," she said.

"It's a rollercoaster of deep trauma that no one should have to experience."

But the threats to MPs have not stopped, especially to women.

Former Conservative leader Iain Duncan Smith said that on Monday, his staff had received a newspaper front-page featuring Amess and daubed with the handwritten words "like you, you bastard".

Labour MP Chris Bryant said he notified police of another death threat on Saturday, after having urged more civility in politics.

Bryant attributed the rise in abuse to Brexit and anti-vaccine protests.

Closing the debate, Labour's Rupa Huq dwelt fondly on sharing a recent foreign trip to Qatar -- Amess' last as an MP -- and urged colleagues to follow his example in "being less cross and more cross-party".

Last month, Labour's deputy leader Angela Rayner triggered a storm of protest after describing Conservatives as "scum".

But critics have also long condemned Johnson's provocative language against a range of targets in print and in speeches.

Police have until Friday to charge the 25-year-old man who was arrested at the scene.

Detectives have declared the killing a terrorist incident and said they were investigating "a potential motivation linked to Islamist extremism".

British media, citing official sources, have identified the suspect as Ali Harbi Ali, a British national of Somali descent from London.

Before the attack, the suspect had been referred to Prevent, the official counter-terrorist scheme for those thought to be at risk of radicalisation, according to reports.

His father is a former prime ministerial adviser in Somalia and his uncle is the East African country's ambassador to China, while his aunt runs a security think tank in the war-ravaged Somali capital Mogadishu, the reports added.