The attacker who rammed pedestrians and stabbed a police officer in central London was a British-born man once investigated for "violent extremism," Prime Minister Theresa May said yesterday.

"The case is historic. He was not part of the current intelligence picture," May told lawmakers as the House of Commons reopened following a lockdown on Wednesday afternoon.

The Islamic State extremist group later claimed that the attacker, who killed three people and injured 29, was one of its "soldiers".

Police identified him as Khalid Masood, 52, and said he used several aliases and most recently lived in the city of Birmingham in the West Midlands.

Authorities had "no prior intelligence about his intent to mount a terrorist attack," though Masood had several convictions for assault and other offences, the Metropolitan Police said.

May told parliament earlier that the attacker was "once investigated in relation to concerns about violent extremism," adding that he was "a peripheral figure".

"Police detained eight people in raids at several addresses in London and Birmingham in connection with the deadly rampage.

Masood drove a car into pedestrians on central London's Westminster Bridge before going after officers with a knife. He killed three people, including one officer, before being shot dead. A fourth person, a 75-year-old man, died yesterday in hospital.

Pedestrian and vehicle traffic resumed across the bridge early yesterday afternoon, nearly 24 hours after the attack.

Hundreds of lawmakers were kept inside the parliament building for up to five hours immediately after the attack.

The assailant acted alone and was inspired by international terrorism, London's Metropolitan Police Service said.

Islamic State's semi-official Amaq agency said in an online claim that the attack was in response to the radical group's call for targeting citizens of countries participating in the fight against it.

An international US-led alliance is carrying out an air campaign against Islamic State in Syria and neighbouring Iraq.

May said that those injured in the Westminster attack included 12 Britons, three French children, two Romanians, four South Koreans, two Greeks, and one each from Germany, Poland, Ireland, China, Italy and the United States. Three police officers were also wounded.

Before May's speech, lawmakers held a minute's silence for the victims of the terrorist attack.

It began at 9.33am (5.33pm Malaysian time) to signify the 933 badge number of the parliamentary protection officer who died in the attack, 48-year-old Keith Palmer.

May paid tribute to the "extraordinary efforts" to save Palmer's life, highlighting the role of Conservative lawmaker Tobias Ellwood, who gave the stricken officer mouth-to-mouth resuscitation.

She said people across Britain were "going about their days and getting on with their lives" as normal.

"It is in these actions - millions of acts of normality - that we find the best response to terrorism," May said.

"My thoughts, prayers, and deepest sympathy are with all those who have been affected by yesterday's awful violence," Queen Elizabeth II said, praising the police response to the attack.

Seven of the 29 injured remain in critical condition at hospital, police said.

- dpa