London attacker British-born, had been investigated over extremism concerns

The attacker who killed three people near the British parliament before being shot dead was British-born and was once investigated by MI5 intelligence agents over concerns about violent extremism, Prime Minister Theresa May said on Thursday.

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 religious ideology. Forty people were injured and 29 remain in hospital, seven in critical condition.

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.

"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 the 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.

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.

It was the worst such attack in Britain since 2005, when 52 people were killed by militant Muslim suicide bombers on London's public transport system. Police had given the death toll as five but revised it down to four on Thursday.

The casualties included 12 Britons, three French children, two Romanians, four South Koreans, one German, one Pole, one Chinese, one American and two Greeks, May said.

Daesh claims responsibility for British parliament attack

Daesh was responsible for an attack outside Britain's parliament which left four people dead, the group's Amaq news agency said on Thursday.

"The perpetrator of the attacks yesterday in front of the British parliament in London is a Daesh soldier and he carried out the operation in response to calls to target citizens of the coalition," the Amaq statement said.

Daesh, which has controlled parts of Iraq and Syria in recent years, has lost territory this year to local forces in those countries supported by a US-led military coalition.