The House of Commons has been suspended following reports of a terrorist attack outside Parliament.
Parliament was under lockdown after a police officer was stabbed by an assailant, who was then shot by police. Officers said police were also called to an incident on Westminster Bridge nearby.
David Lidington, leader of the House of Commons, told MPs: "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 it would be wrong for me to go into further details until we have confirmation from the police and house security authorities about what is going on.
"I shall endeavour to do the very best I can, both at the dispatch box and by communicating with my opposite numbers in other political parties to ensure members are kept aware of what is happening.
"But at the moment the very clear advice from the police and director of security in the house is that we should remain under suspension and that the chamber should remain in lockdown until we receive advice that it is safe to go back to normal procedures."
Shadow leader of the house Valerie Vaz added: "Our thoughts and prayers are with the police officer. [I want] to thank the police and all security services, and all the staff for looking after us so well."
On the bridge, witnesses said a vehicle struck several people, and photos showed a car ploughed into railings. Witnesses in Parliament reported hearing sounds like gunfire.
Eyewitness video has shown five people being “mown down” by a car on Westminster Bridge.
Radoslaw Sikorski posted a video to Twitter purporting to show people lying injured in the road on Westminster Bridge.
Mr Sikorski, a senior fellow at Harvard Centre for European Studies, said he saw at least five people lying on the ground after being “mown down” by a car.
“I heard what I thought what I thought was just a collision and then I looked through the window of the taxi and someone down, obviously in great distress,” he told the BBC.
“Then I saw a second person down, and I started filming, then I saw three more people down, one of them bleeding profusely.”
Police have said they are treating the attack as a terrorist incident and asked people to avoid the immediate area to allow emergency services to deal with the ongoing incident.
