British police confirmed that deal toll in Wednesday's terror attack outside the Parliament in central London has risen to five.

On Wednesday, three people and a terrorist were dead after an attacker brought carnage to central London on Wednesday, mowing down pedestrians on Westminster Bridge and hacking at police with knives in the grounds of the Houses of Parliament.

Metro Police Assistant Commissioner Mark Rowley told reporters that about 40 people were injured in the attack.

Rowley, also London's top anti- terror officer, said that the officer who died in the attack has been named as Keith Palmer, who was 48 and had 15 years service as a police officer.

He said he would not comment on the identity of the attacker, although he suggested the police was aware of who the assailant was.

The attacker was shot dead after stabbing Palmer. The other three victims were all civilians, the police officer said.

At least 40 people were hit on the crowded bridge when a 4x4 drove along the pavement, knocking down pedestrians before crashing into a fence below Big Ben, the local media reported.

An intruder, described by a witness as "middle-aged and Asian", then managed to break into the grounds of the Parliament where he fatally stabbed a police officer with two knives, the media reported.

The policeman died at the scene. The attacker, who was shot by armed officers, died after he was taken to hospital.

Prime Minister Theresa May was bundled into her car by a plain-clothes police officer and driven quickly from the scene as the attack unfolded. She will chair a meeting of the Government's emergency Cobra Committee.

Scotland Yard said the attack, which comes a year to the day after the terrorist atrocities in Brussels, was being treated "as a terrorist incident until we know otherwise".

Witness Jayne Wilkinson said: "We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.

"And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.

"He had a lightweight jacket on, dark trousers and a shirt. He was running through those gates, towards Parliament, and the police were chasing him."

A spokesman for the Port of London Authority, which looks after safety on the River Thames, said: "A female member of the public was recovered from the water near Westminster Bridge. She is alive but undergoing urgent medical treatment on a nearby pier. We believe she fell from the bridge".

He said the river has been closed from Vauxhall to Embankment "as part of the security response".