Police officer 'stabbed', 'assailant' shot outside Parliament
The building in the heart of the British capital was immediately sealed off and MPs and staff ordered to remain inside.
David Lidington, the leader of parliament's lower House of Commons who is responsible for arranging government business, told MPs: "What I am able to say to the house is there has been a serious incident.
"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.
"There are also reports of further violent incidents in the vicinity of the Palace of Westminster," he said.
He said he could not go into further details until there was confirmation from the police and security authorities.
Prime Minister Theresa May is safe, a statement from her Downing Street office said, and she was seen getting into a car and being driven away from parliament.
Lidington said an air ambulance was at the scene to take away casualties.
TV pictures showed traffic halted on the nearby Westminster Bridge and emergency vehicles swarming around. The busy bridge was completely shut off to traffic.
Armed police swarmed to the area which was quickly sealed off to the public.
The incident came on the same day that Belgium was marking a year since its most deadly attack, when 32 people were killed in suicide bombings at Brussels Airport and a metro station.
In July 2005, four British suicide bombers inspired by Al-Qaeda attacked London's transport system during rush hour, killing 52 people.
Two weeks later, there was an attempt to carry out a second wave of attacks.
A spokesman for the House of Commons told AFP:
"We can confirm that the sitting has been suspended in the Commons at the moment. We are aware of a security incident that has taken place."
London's Metropolitan Police said on Twitter: "We were called at approx 2:40 pm to reports of an incident at Westminster Bridge. Being treated as a firearms incident -- police on the scene.
"Officers -- including firearms officers -- are on scene and dealing with the incident."
A staff member in parliament, who did not want to be named, told AFP: "I definitely heard shots. I saw someone in dark clothing go down."
Westminster station on the London Underground train network, which is opposite the Houses of Parliament, was closed over to the incident.
JOIN OUR PULSE COMMUNITY!
Eyewitness? Submit your stories now via social or: