UK parliament attacker British-born, had been investigated over extremism concerns

The Islamic State group claimed responsibility for the attack in a statement issued by its Amaq news agency. But it gave no name or other details and it was not clear whether the attacker was directly connected to the group.

Police arrested eight people at six locations in London and Birmingham in the investigation into Wednesday's lone-wolf attack that May said was inspired by a warped Islamist ideology.

UK PM Theresa May on her way to House of Common. Photo: Reuters

About 40 people were injured and 29 remain in hospital, seven in critical condition, after the incident which resembled Islamic State-inspired attacks in France and Germany where vehicles were driven into crowds.

The assailant sped across Westminster Bridge in a car, ploughing into pedestrians along the way, then ran through the gates of the nearby parliament building and fatally stabbed an unarmed policeman before being shot dead. 

Investigation agency at the scence. Photo: Reuters

"What I can confirm is that the man was British-born and that some years ago he was once investigated by MI5 in relation to concerns about violent extremism," May said in a statement to parliament.

"He was a peripheral figure...He was not part of thes current intelligence picture. There was no prior intelligence of his intent or of the plot," she said, adding that his identity would be revealed when the investigation allowed.

The mayhem in London took came on the first anniversary of attacks that killed 32 people in Brussels. Twelve people were killed in Berlin in December when a truck ploughed into a Christmas market and 84 died in July in a similar attack on Nice waterfront for which Islamic State claimed responsibility.

Islamic State, which is being driven from large areas of Iraq and Syria by local forces supported by a U.S.-led military coalition, said it was responsible for the London attack.

"The perpetrator of the attacks...is an Islamic State soldier and he carried out the operation in response to calls to target citizens of the coalition," a statement on its Amaq agency said.

Westminster Bridge and the area just around parliament were still cordoned off on Thursday morning and a line of forensic investigators in light blue overalls were on their hands and knees, examining the scene where the attacker was shot.

The dead were two members of the public, the stabbed policeman and the attacker.

"My thoughts, prayers, and deepest sympathy are with all those who have been affected by yesterday's awful violence," Queen Elizabeth said in a message.

Britain's plan to trigger the formal process of exiting the EU on March 29 will not be delayed due to the attack, May's spokesman said.