Witnesses are describing scenes of chaos and terror following attacks outside the U.K. Parliament and on the nearby Westminster Bridge in London.

The attacks, which left four dead, including the assailant, occurred Wednesday in a very busy area popular with tourists. A lone suspect plowed down pedestrians on Westminster Bridge, killing two and injuring 40, before fatally stabbing a police officer on Parliament’s grounds. The attacker was shot to death by police at the scene.

Camilla Tominey, a Sunday Express reporter, told CTV News Channel that she was having coffee in a building near the Parliament when she heard “a huge commotion.”

She then saw police officers yelling at people to get back and hide.

Another witness, Natasha Monet, said she saw a grey SUV crashed into a fence near the entrance to Parliament grounds and a swarm of officers surrounding it.

Monet, who works in a building across the street, told CTV News Channel that the scene was “extremely shocking,” but not necessarily surprising.

“We have been warned in London that a terrorist attack was likely,” she said.

Rick Longley told the Press Association that he saw a man stab a policeman outside Parliament.

"We were just walking up to the station and there was a loud bang and a guy, someone, crashed a car and took some pedestrians out," he said.

"They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben. A guy came past my right shoulder with a big knife and just started plunging it into the policeman.

"I have never seen anything like that. I just can't believe what I just saw."

Authorities in London said the attacker was shot by police on the scene.

William James, a political correspondent with Reuters, said he was sitting in his office on Parliament grounds when he and his colleagues heard a loud bang outside.

“And that was immediately followed by a number of gunshots,” he told CTV News Channel.

When he looked out the window, James saw armed police everywhere and paramedics treating two people on the ground.

He said emergency personnel eventually stopped treating one person and loaded the other individual into an ambulance.

Meanwhile, a car plowed into pedestrians on the nearby Westminster Bridge.

A woman who was on a passing bus at the time said she heard what she believed to be gunshots and then saw a car strike a woman.

“I could hear screams and then we heard gunshots again and as we looked along the bridge…there was bodies…at least 10, 12 bodies in different places. It was horrendous, absolutely horrendous,” she told reporters.

Another man who witnessed the attack on the bridge, Russell Williams from Toronto, was in a cab just a few blocks away from the scene of the attack. He recalled sitting in the cab behind a bunch of buses before they moved up and he saw the commotion.

“Next thing you know I seen a lot of people looking over towards the bridge, a lot of people crying, then we moved up some more and I watched the one helicopter land on the bridge,” Williams told CTV Toronto after he landed at Pearson Airport on Wednesday evening.

Williams said he didn’t know what was happening until he turned on the radio.

A crowd of parents at Vancouver International Airport expressed their relief when their children landed safely after their flight from London on Wednesday. The group of 42 students from St. George’s School were in the U.K. playing rugby. One father at the airport told CTV Vancouver that there wasn’t much he could do back home but hope the students have a good time and never experience something as awful as Wednesday’s attack.

British Prime Minister Theresa May condemned the incident as a “sick and depraved terrorist attack” on Wednesday. The threat level for terrorism in the U.K. was already listed as severe, meaning an attack was “highly likely.”

With files from The Associated Press