Westminster attack: Car-and-knife rampage kills four

Scenes of carnage on Westminster Bridge near parliament in London after car-and-knife attack also wounds at least 20.

Four people were killed and at least 20 wounded in London on Wednesday after a car ploughed into pedestrians and an attacker went on a stabbing spree with a knife before being shot dead close to the British parliament.

London Metropolitan Police Commissioner Mark Rowley told reporters a "full counter-terrorism investigation" into the attack was under way.

He said the violence started when a car was driven over Westminster Bridge, hitting and injuring a number of civilians and three police officers on their way back from a commendation ceremony.

The car then crashed close to parliament where at least one man armed with a knife continued the attack and tried to enter the building. 

Many people were lying on the ground - some bleeding heavily and apparently unconscious - on Westminster Bridge near Britain's parliament.

Some of the wounds suffered by the victims were described as "catastrophic". One woman was pulled out alive from the River Thames with serious injuries by port authorities.