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A woman has died and a police officer has been stabbed in the Houses of Parliament in London, in what police are treating as a terrorist incident, BBC reports.

The attacker, who was shot by police officers, is reported to have earlier mowed down several pedestrians as he drove a car across Westminster Bridge.

He crashed it into railings before running into the Palace of Westminster and stabbing the officer.

A Downing Street source said Prime Minister Theresa May was safe.

Prime Minister Theresa May was seen being ushered into a silver Jaguar car as what sounded like gunfire rang out at Parliament during the incident.

MPs said they had heard three or four gunshots and staff inside Parliament were told to stay inside their offices.

Commons Leader David Lidington told MPs the "alleged assailant was shot by armed police".

Tom Peck, political editor for the Independent, tweeted: "There was a loud bang. Screams. Commotion. Then the sound of gunshots. Armed police everywhere."

Transport for London said Westminster underground station has been shut at the police's request, and buses diverted.