The shocking attack killed a woman, according to a junior doctor, while police are treating it as a terrorist incident until they "know otherwise".
Reports suggest that the attacke mowed down about a dozen people on Westminister Bridge with a car before going on to attack the police officer.
After causing canage on the pavement of the bridge, the car responsible was seen driving into the railings at Parliament Square.
A knifeman then reportedly ran through the gates into the front yard of the parliamentary compound.
Two people were seen being treated on the ground in New Palace Yard amid shouts and screams.
Prime Minister Theresa May was bundled into a car by plain-clothed police officers and taken to safety. It is known that Mrs May is safe.
London Terror Attack: LIVE IMAGES Wed, March 22, 2017
Two people shot outside Parliament as car mows down 10 people in Westminster
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Emergency services at the scene outside the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament
London terror attack: What we know so far about shooting at the Parliament in Westminster
The House of Commons has been suspended for the day, while MPs are being held inside the building.
An air ambulance later landed in Parliament Square as paramedics rushed to help multiple casualities.
Footage of the Westminister bridge shows at least four injured people lying on the pavement where police condons have been set up.
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More to follow.