"The location of this attack was no accident. The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech," Prime Minister Theresa May said.
The area around Westminster was teeming with people when the attack began at about 2:40 p.m. Witnesses said the attacker drove his car over Westminster Bridge and into the crowds. The vehicle hit a large number of people, including three police officers.
"The car then crashed near to Parliament and at least one man -- armed with a knife -- continued the attack, trying to enter Parliament," said Rowley.
As the incident unfolded outside, the Leader of the House of Commons, David Lidington, told MPs a serious incident had taken place "within the estate." Proceedings were suspended.
Lidington told MPs that it appeared a police officer has been stabbed and that the assailant had been shot.
Conservative MP Damian Collins told CNN that he was walking through an underground tunnel from his office to vote in the chamber with about 30 other MPs when he heard shouting followed by gunshots. Police shouted for the MPs to get down on the ground.
"Clearly, something serious had happened but no one was being told what was going on," he said. "The instant response from the police was incredible -- they seemed on top of the situation straight away."
A member of the government, Foreign Office Minister Tobias Ellwood, was pictured giving assistance to a victim in New Palace Yard, a courtyard within the grounds of Parliament.
A fellow Conservative Party MP, Maria Miller said he had "given mouth-to-mouth resuscitation to one victim," the Press Association said. A witness told CNN that Ellwood was trying to save the police officer, who died.
Police put the Westminster area on lockdown, securing roads, Parliament buildings and the Westminster underground station. The zone was soon expanded to a larger part of central London, including the London Eye Ferris wheel, which was halted for hours with people still on it. They have since been let off.
The London Ambulance Service treated at least 10 patients on Westminster Bridge.
In the chaos, a woman was pulled from the River Thames alive, but with serious injuries.
The Prime Minister called a meeting of COBRA, the UK's emergency Cabinet committee.
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