'Soldier of Islamic State' carried out London attack

Islamic State has claimed responsibility for an attack outside Britain's parliament that left four people dead, the group's Amaq news agency said.

A British-born man mowed down pedestrians on the popular Westminster Bridge, killing mother-of-two Aysha Frade and a man aged in his 50s, before stabbing police constable Keith Palmer to death outside parliament.

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

The Westminster crime scene is being cleaned. Picture: AAP

Islamic State, 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 U.S.-led military coalition.

British Prime Minister Theresa May confirmed the London attacker had been on the intelligence radar previously over his “violent extremism”, but said that was a number of years ago.

A tribute for slain officer constable Keith Palmer. Picture: AAP

“Some years ago, he was once investigated by MI5 in relation to concerns about violent extremism,” she told parliament on Thursday.

“The case is historic, he was not part of the current intelligence picture.

“There was no prior intelligence of his intent or the plot.”

Theresa May leaves 10 Downing Street under guard. Picture: AAP

Ms May said the UK would not bend to any terror threat as she passed on her condolences to the three killed and the 29 wounded in the attack.

“As I speak millions will be boarding trains and aeroplanes to travel to London and to see for themselves the greatest city on earth,” she said.

The London terror scene. Picture: AAP

“It is in these actions millions of acts of normality that we find the best response to terrorism.

“It’s a response that denies our enemy their victory … that shows we will never give in.”

The attacker was shot dead by officers outside parliament after he killed Constable Palmer.

London terror victim Aysha Frade. Picture: Facebook

Eight people have now been arrested in subsequent raids carried out across London, Birmingham and other suburbs.

The attacker is yet to be publicly named.