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An eerie soundtrack of helicopter engines, sirens and police radios cloaked the historic Palace of Westminster, home of Britain's Houses of Parliament, as tourists, journalists and lawmakers ran for cover after what police called a terrorist attack Wednesday.

"We saw loads of people ducking down," George Nuth, 21, who was walking toward nearby Westminster Abbey at the time of the attack, told NBC News. "It must've been a tour group, around 20 or 30 people, and police or security were telling people to run. A policeman said to us: 'Stop dawdling if you don't want to get shot.'"

Four people were killed and approximately 40 others were injured before the assailant was himself shot and killed, police said.

Members of Parliament, or MPs, were locked inside the House of Commons as police secured the scene. Tourists were trapped on the London Eye, the enormous celebratory Ferris wheel on the nearby South Bank of the River Thames. Authorities raced to fish a seriously injured woman out the Thames, into which she had fallen from Westminster Bridge.

Radoslaw Sikorski, a former war correspondent and onetime foreign minister of Poland, said he watched as pedestrians were knocked to the ground by the vehicle the attacker used to plow into bystanders on the bridge. He tweeted that a car had just "mowed down at least 5 people."

A car on Westminster Bridge has just mowed down at least 5 people. pic.twitter.com/tdCR9I0NgJ — Radoslaw Sikorski (@sikorskiradek) March 22, 2017

Steve Voake, 55, a children's book author who was visiting London for the day, told NBC News that he was walking over the bridge when he heard screaming.

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"I thought it was a road accident. There was lots of panic and confusion. Then I saw a shoe on the ground. I one body on one side of the road and one body face down in the water with blood all around it."

Police in bright fluorescent jackets shepherded tourists, officers, journalists and lawmakers away from the scene as they emptied the streets around Parliament block by block."We were obviously prepared for the worst," said Chris Law, a Scottish National Party MP. "We've been under severe threat for some time, [so] we knew what to do."

Still, he said, he shared everyone's sense of "shock and horror."

Daniel Velasco, an 18-year-old medical student, happened on the scene as he emerged from the nearby Westminster subway station.

"I saw someone being put on a stretcher; then a helicopter landed," Velasco told NBC News. "The police told me this guy had a knife — that's why they shot him."

Journalist Quentin Letts told MSNBC that he had witnessed one of the incidents from his office.

"I saw a fairly thick-set man wearing black clothes running through the open gates, the security gates where people drive their cars," Letts said. "He seemed to have something in his hand, maybe a knife, maybe a stick."

Letts added: "He started beating a policeman who had fallen over on the ground. The policeman managed to shake him off, and the attacker then ran towards the entrance of the House of Commons ... and he got about 15 yards before the authorities responded."

Letts said police officers acted "incredibly fast."

Camilla Tominey, a journalist who works for Britain's Express newspaper and was also inside the complex, told MSNBC that she heard gunshots before "police told us to 'run, run, run.'"

As they scoured the area hours later, officers mourned the loss of one of their colleagues, whom the attacker stabbed to death, despite the best efforts of doctors and a passing government minister.