A police officer has been stabbed in the Houses of Parliament in central London, Commons Leader David Lidington has said.
He told MPs the "alleged assailant was shot by armed police" following a "serious" incident.
Staff inside Parliament were told to stay inside their offices.
The BBC's Laura Kuenssberg said police told her someone had been shot and MPs said they heard "three or four gunshots".
Tom Peck, political editor for the Independent, tweeted: "There was a loud bang. Screams. Commotion. Then the sound of gunshots. Armed police everywhere."
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.
A Downing Street source confirmed that Prime Minister Theresa May was "OK".
Scotland Yard said it was called to a firearms incident on Westminster Bridge amid reports of several people injured.
Transport for London said Westminster underground station has been shut at the police's request, and buses diverted.
Mr Lidington said: "It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.
"An air ambulance is currently attending the scene to remove the casualties.
"There are also reports of further violent incidents in the vicinity of the Palace of Westminster but I hope colleagues on all sides will appreciate that it'd be wrong of me to go into further details until we have confirmation from the police and from the House security authorities about what is going on."
An eye witness, Radoslaw Sikorski, a senior fellow at Harvard's Centre for European Studies, posted a video to Twitter purporting to show people lying injured in the road on Westminster Bridge.
He wrote: "A car on Westminster Bridge has just mowed down at least 5 people."