Anti-terror police search a property allegedly linked with the man suspected of murdering Sir David Amess in London, the UK. Photo: AFP

The family of murdered British MP David Amess said on Sunday they were "completely broken" by his death, but that hatred had to be set aside as they made a plea for "togetherness."

Veteran Conservative lawmaker Amess, who was 69, was stabbed to death on Friday as he met voters at a church in Leigh-on-Sea, east of London, in the second such attack on an MP in just over five years.

Police have declared the crime a terrorist incident and said they are investigating "a potential motivation linked to Islamist extremism." 

They have detained a 25-year-old man under the Terrorism Act, which allowed them to extend his detention for questioning until Friday. He has not been charged.

"We are absolutely broken, but we will survive and carry on for the sake of a wonderful and inspiring man," the bereaved family said in a statement.

Detectives said late Saturday they were searching three addresses in the London area in a "fast-paced investigation."

On Sunday, police stood on guard in front of a three-story house in a quiet street in the north London district of Kentish Town. 

A woman living nearby told AFP she did not think the suspect lived there.

"We know the family, they are lovely people. They have three sons but they are older" (than the suspect), said the woman, who did not give her name.

British media, citing unnamed official sources, identified the suspect as Ali Harbi Ali, a British national of Somali descent who had been referred to Prevent, the official counter-terrorist scheme for those thought to be at risk of radicalization.

Ali was believed not to have spent long on the program, which is voluntary, and was never formally a "subject of interest" to MI5, the domestic security agency, the BBC said.

The suspected attacker remained at the scene after stabbing Amess multiple times, British media reported.

The Amess family in their statement said: "We are trying to understand why this awful thing has occurred. Nobody should die in that way. Nobody. Please let some good come from this tragedy."

Police and security services believe the suspect acted alone and was "self-radicalized," The Sunday Times said, while adding he may have been inspired by Al-Shabaab, Al-Qaeda-linked Islamists in Somalia.

Ali's father, named as Harbi Ali Kullane and said to be a former adviser to the prime minister of Somalia, confirmed to The Sunday Times that his son was in custody, adding: "I'm feeling very traumatized." 

Interior minister Priti Patel said the Prevent program that reportedly had the suspect in its sights is currently under independent review.

AFP