British police say five people died in the terror attack outside Parliament.
Counter-terrorism chief Mark Rowley said one policeman, three civilians and the attacker died. He said a further 40 people were wounded.
The assailant has not been identified. Rowley said police think they know the identity of the man but would not reveal details. He said Islamic extremism is suspected in the attack.
He said extra armed police would be on the streets in the coming days to reassure the public, and hundreds of police officers are working on the case.
He identified the police officer who died as Keith Palmer, 48.
Here's how events unfolded:
• First reports emerge of loud bangs or gunshots outside the Houses of Parliament, the seat of the UK government in the heart of the Westminster area.
• The Metropolitan Police say they are investigating reports of an incident in Westminster. A short time later they tweet that it is being treated as a firearms incident.
• The Deputy Speaker of the House of Commons, Lindsay Hoyle, suspends the sitting at 2:44 p.m. local time and tells Members of Parliament they should wait in the Chamber.
• Fifteen minutes later, David Lidington, the Leader of the House of Commons, says the alleged assailant was shot by police and an air ambulance is at the scene to remove the casualties. "We should remain under suspension and the chamber should remain in lockdown until we receive advice it is okay to go back to normal procedures," Lidington adds.
• People posting on Twitter describe a car "collision" on Westminster Bridge and say they can see people lying dead or injured on the road. Images emerged of a car crashed into the perimeter gate of Parliament, and witnesses later told CNN that they saw the vehicle mow down pedestrians.
• Police clear the area, which is popular with tourists as well as being home to many government offices. Westminster Underground station is closed and bus routes are diverted away from the area.
• Britain's Press Association news agency reports that Prime Minister Theresa May was seen being ushered into a car and driven away from the area.
• Police say they are treating the incident as an act of terrorism, "until they know otherwise."
• Met Police Commander B.J. Harrington then tells reporters outside Scotland Yard that "a full counter terrorism investigation is underway," although police remain open-minded on the motive. He adds: "We received a number of calls, which included a person in the river, a car in a collision with pedestrians and a man that was armed with a knife." Harrington says there are "a number of casualties including police officers" but he cannot give details of their injuries.
• Security will be stepped up across London as people travel home from work, Harrington says, with more armed and unarmed officers on patrol. He urges Londoners to be vigilant.
• London Ambulance Service says at least 10 people have been treated on Westminster Bridge and that a number of hospitals have been placed on alert.
• Theresa May is to chair a meeting of the Government's emergency Cobra committee to discuss the immediate response to the Westminster incident, Downing Street says. The Cobra committee brings together government ministers with senior officials of the emergency services and security and intelligence agencies.
• A woman has been pulled from the Thames near Westminster Bridge alive but with injuries, Port of London Authority spokesman Martin Garside tells CNN. "I don't know how she got into the water," he said. She is undergoing urgent medical care for her injuries.