LONDON -- Witnesses said a heavy-set man wearing black clothes and brandishing a big knife attacked a police officer on the grounds outside Britain’s Parliament Wednesday, sending the compound into lockdown. The attacker and the officer both later died, police said.

Daily Mail reporter Quentin Letts told CBSN that the attacker ran through a gate where cars enter the parliamentary estate. Letts described the man as “thick-set” and dressed in black.

“He then started striking a policeman who was on the ground,” Letts said.

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Rick Longley told Britain’s Press Association that he saw a man stab an officer.

“A guy came past my right shoulder with a big knife and just started plunging it into the policeman,” Longley said. “I have never seen anything like that. I just can’t believe what I just saw.”

Letts told CBSN the attacker then ran toward an entrance used by members of Parliament.

“He got about 15 yards, I guess, before two plainclothes officers with guns shouted out and warned him and then about three shots, and the attacker fell,” Letts said.

The head of counterterrorism at London’s Metropolitan Police, Mark Rowley, later said that the attacker and officer had died.

British lawmaker Grant Shapps said on Twitter that he was walking through the cloisters of the House of Commons to vote when he heard four gunshots. Police told lawmakers to get down on the ground and crawl to cover.

Police response instant. Heard commotion, looked round. Police weapons drawn, 4 shots, police ordered us to hit ground & get back, get back. — Grant Shapps (@grantshapps) March 22, 2017

“Police response instant. Heard commotion, looked round. Police weapons drawn, 4 shots, police ordered us to hit ground & get back, get back,” he tweeted.

Around the same time, a vehicle mowed down pedestrians on nearby Westminster Bridge, killing at least two other people and injuring around 20 others. Authorities said they were treating the attack as a “terrorist incident until we know otherwise.”

Police believe there was only one attacker. There was no immediate claim of responsibility.

Poland’s former foreign minister, Radek Sikorski, said he was in a taxi leaving Westminster and was checking his email when he heard something like a car crash, “something like a car hitting metal sheet.”

“I look up and I see that a person is lying on the pavement. I started my camera and I saw more people lying on the street and on the pavement,” Sikorski said on Poland’s TVN24.

“People started running up to them. I saw one person who gave no sign of life, another man was bleeding from his head. In all, I saw five people who were at least seriously injured,” he said.

“The taxi driver immediately called the emergency number. I heard ambulances within two or three minutes, so the rescue action was really very quick. There is a hospital near there.”

“It all happened so fast that there was no time to get scared,” said Sikorski who posted his video on Twitter.