LONDON — Four people were confirmed dead Wednesday after Britain's Parliament was locked down Wednesday following a "terrorist incident" involving a 4x4 that mowed down pedestrians.

At least 20 others were injured when the vehicle hit people walking on nearby Westminster Bridge before crashing into a railing.

A police officer was also stabbed near the House of Commons. Pictures from the scene showed a junior government minister helping doctors treat the officer.

Authorities later confirmed that the police officer, two civilians and the suspected attacker were dead.

A woman was pulled alive from the River Thames with serious injuries, port officials told NBC News.

The Associated Press quoted a hospital official as saying that a female pedestrian was among those killed.

"There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries," said Colleen Anderson, who is a junior doctor at St. Thomas' Hospital.

Finance Minister Philip Hammond described it as a "horrific attack."

Journalists based at the Parliament reported hearing gunshots shortly after 2:30 p.m. local time (10:30 a.m. ET) and were told to stay in their offices.

A former government minister, Grant Shapps, said police had ordered lawmakers to crawl for cover as the incident unfolded. Some tweeted from inside the House of Commons debating chamber where they were held while police secured the scene.

Eyewitness Daniel Velasco, 18, a medical student, saw the aftermath from nearby Westminster subway station.

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

Journalist Quentin Letts, who was inside Parliament, 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 had acted "incredibly fast."

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

George Nuth, 21, told NBC News he was walking towards nearby Westminster Abbey at the time of the attack.

"We saw loads of people ducking down," he said. "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.'"

Radoslaw Sikorski, a former Polish lawmaker, said he had seen pedestrians knocked down by the vehicle on Westminster Bridge.

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

"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."

Streets around nearby Downing Street, which serves as both the home and office of British Prime Minister Theresa May, were evacuated.

French Prime Minister Bernard Cazeneuve said high school students from his country were among the injured. In a tweet Wednesday, he offered support to the British as well as to "the French students wounded, their families and their schoolmates."

The incident comes on the first anniversary of the terror attacks in the Belgian city of Brussels that killed 32 people.

Lawmaker Barry Sheerman tweeted a picture from inside the House of Commons during the lockdown.

Bill Kearns, 58, from New York, was visiting a nephew who works in the Parliament.

"We were walking up Embankment heading towards Parliament and then there were lots of people starting to run towards us," he told NBC News. "We tried to get a hold of our nephew straight away, we were worried about him, texting him … and he said: 'Get away from Parliament as fast as you can.'"