Westminster attack: Seven arrested in British police raids

The UK's top anti-terror officer Mark Rowley also said 29 people were treated in hospital, including seven who are still in critical condition, following Wednesday's assault on the symbol of the country's democracy.

"We have searched six addresses and made seven arrests," Rowley told reporters, adding that the raids included locations in London and the central city of Birmingham.

"It is still our belief... that this attacker acted alone yesterday and was inspired by international terrorism," he said. 

Rowley said the victims were a police officer stabbed to death by the attacker at the gates of parliament and two members of the public who were mown down by his car on nearby Westminster Bridge moments before, revising down an earlier toll of four victims.

Among the injured were three French school children and a number of foreign tourists.

Prime Minister Theresa May described the attack in the heart of London as "sick and depraved" in a an address on Wednesday.

"We will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart," said May.

Defence Minister Michael Fallon told BBC radio that Wednesday's carnage was linked to "Islamic terrorism in some form".

Hundreds of extra police were on patrol and officers worked around the clock to piece together what happened in the deadliest attack in the UK since four suicide bombers killed 52 people on London's transport system in July 2005.

The British flag over parliament flew at half-mast.

London's mayor Sadiq Khan called a candlelit vigil on Trafalgar Square later in the day on Thursday.

Agencies contributed to this report.