Footage has emerged of the moment police opened fire on a suspected terrorist outside London's Houses of Parliament.

The suspected assailant was shot after one police officer was fatally stabbed. Another woman died after she was mown down by a car on Westminster Bridge.

At least 11 other pedestrians were hit by the vehicle, according to a junior doctor at St Thomas' Hospital. Some were described as having "catastrophic'' injuries.

One video shows multiple casualties strewn across the road and pavement on Westminster Bridge after a 4x4 reportedly ploughed into pedestrians. Emergency service vehicles can be seen flocking to the scene as members of the public try to help the injured.

People lying on road after being ploughed by vehicle in Westminster

Witnesses described their horror as the chaotic scenes unfolded, causing Parliament to be suspended and Westminster to be put on lockdown.

Rick Longley said he saw a man stab a policeman outside Britain's Parliament.

"We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out," he said. "They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben.

Westminster terror incident: Inside the House of Commons in lockdown

"A guy came past my right shoulder with a big knife and just started plunging it into the policeman.

"I have never seen anything like that. I just can't believe what I just saw."