Four people, including an armed police officer and a man believed to be the attacker, have died in a terrorist incident near the Houses of Parliament.
One of the victims was among several pedestrians hit by a car on Westminster Bridge, before it crashed.
An officer protecting Parliament was stabbed by an attacker, who was then shot by police.
At least 20 people were injured, including three other officers. The attacker has not yet been identified.
Acting Deputy Commissioner and head of counter terrorism at the Metropolitan Police, Mark Rowley, said a major investigation was under way into the “marauding terrorist attack”.
He said the attack started shortly before 15:00 GMT when the car was driven over Westminster Bridge, hitting and injuring a number of members of the public and the three police officers, who were on their way back from a commendation ceremony.
Mr Rowley said after the car crashed, a man armed with the knife “continued the attack and tried to enter Parliament”.
He said it was believed there was only one attacker, but “I am sure the public will understand us taking every precaution in locking down and searching the area as thoroughly as possible”.
In latest developments:
- Prime Minister Theresa May is chairing a meeting of the government’s emergency Cobra committee
- Earlier, a Downing Street spokesman said: “The thoughts of the PM and the government are with those killed and injured in this appalling incident, and with their families.”
- Westminster underground station was shut and remains open for interchange only
- Home Secretary Amber Rudd urged everyone to remain calm but to be vigilant and if they see anything they were concerned about they should report it to the police
- A group of French schoolchildren were also on the bridge and three were injured
- 13 students from Edge Hill University in Lancashire were also caught up in the incident – two were taken to hospital and described as walking wounded; two others had minor injuries
- There are two casualty bureau numbers: 0800 056 0944 and 0207 158 0010 for people worried about family and friends, or eyewitnesses
- Kings College Hospital says eight patients are being treated there – six male, and two female. Two are critical and two are stable
- Around 1,000 people were taken to Westminster Abbey for safety and were then being processed by police
- MPs were locked in the House of Commons for more than four hours and business suspended
- Parliamentary authorities say both the House of Commons and Lords will sit at their usual times on Thursday
- Over the following days there will be extra unarmed and armed officers on the streets of London
Witnesses say there was what appeared to be a warning shout and then the crack of three or four shots before the attacker fell to the floor.