Witnesses say a car mowed down five pedestrians before crashing into the gates outside Portcullis House.
A police officer has reportedly been stabbed.
Deputy Parliament Speaker said that lower chamber House of Commons sitting suspended as witnesses report sounds like gunfire.
Media photographs show people lying on Westminster Bridge, bleeding heavily.
The leader of the House of Commons says an alleged assailant was shot by armed police inside the parliament perimeter.
Police have evacuated the area.
A car on Westminster Bridge has just mowed down at least 5 people. pic.twitter.com/tdCR9I0NgJ
— Radosław Sikorski (@sikorskiradek) March 22, 2017
Witnesses say a car mowed down five pedestrians before crashing into the gates outside Portcullis House.
It has also been reported that a man was seen with a knife.
The parliament building is in lock down. Staff inside parliament have been told to stay inside their offices.
Scotland Yard said it was called to a firearms incident on Westminster Bridge amid reports of several people injured.
An air ambulance has landed in Parliament Square.