British police shot a suspected attacker outside parliament after an officer was stabbed in what police said was a "terrorist" incident.
David Lidington, the leader of parliament's 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.
A car drove into a crowd on Westminister Bridge, injuring at least 12 people, and hit the gates outside parliament.An air ambulance was at the scene to take away casualties.
TV footage showed traffic halted on the nearby Westminster Bridge and emergency vehicles swarming around. The busy bridge was completely shut off to traffic.
| An air ambulance was at the scene to take away casualties [Getty] |
Theresa May was safe and driven away from parliament, according to a statement her Downing Street office.
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.
London's Metropolitan Police said on Twitter: "We are treating this as a terrorist incident until we know otherwise".
Westminster station on the London Underground train network, which is opposite the Houses of Parliament, was closed over to the incident.