LONDON (CNN) -- At least three people died and 20 were injured when an attacker rammed a car into crowds of people before stabbing a police officer in the grounds of the UK Parliament.
A woman was pulled alive from the River Thames amid chaos in the area, the heart of political life in Britain. Among the dead were a police officer and the assailant, police said, announcing a "full counter-terrorism investigation."
Parliament was placed on lockdown and lawmakers were forced to remain in the main debating chamber of the House of Commons as firearms officers rushed to the scene outside.
Pictures showed scenes of carnage in the aftermath of the attack, which happened as Parliament was in session. One member of the British government gave mouth-to-mouth resuscitation to one of those injured.
"This is a day that we had planned for -- that we all hoped would never happen -- but sadly it is now a reality, said Mark Rowley, Britain's most senior counter-terror police officer.
The Leader of the House of Commons said a police officer was stabbed. Many of those hurt had "catastrophic injuries", PA said.
"Officers -- including firearms officers -- remain on the scene and we are treating this as a terrorist incident until we know otherwise," police said.
Latest developments
MPs locked in House of Commons into the evening. Prime Minister Theresa May to chair emergency Cabinet meeting. US President Donald Trump speaks with May by phone. Large section of central London closed off. Gates to Buckingham Palace closed. Social media video shows bodies strewn on Westminster bridge. Mayor of London says city 'will not be cowed by terrorism.'
Incident began near Westminster Bridge
The attack began when a car was driven over Westminster Bridge and plowed into a crowd.
It hit and injured members of the public and three police officers, Rowley, the national lead for Counter-Terrorism Policing, told reporters.
"The car then crashed near to Parliament and at least one man -- armed with a knife -- continued the attack, trying to enter Parliament," he said, confirming three people and the attacker were killed.
A police officer was killed as he was defending Parliament from the attack, he said.
London's Metropolitan Police were called at approximately 2:40 pm to reports of a firearms incident. Proceedings in the House of Commons were suspended with dozens of MPs locked inside.
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.
Mayor of London Sadiq Khan said: "My heart goes out to those who have lost loved ones and to everyone who has been affected," he said, paying tribute to the emergency services.
He announced the deployment of additional armed officers on the streets of the British capital. "We stand together in the face of those who would seek to harm us and destroy our way of life," he added. "Londoners will never be cowed by terrorism."
Police appealed for anyone with photos or video of the incident to contact them.
Prime Minister Theresa May called a meeting of COBRA, the UK's emergency Cabinet committee.
US President Donald Trump spoke with May on the phone after learning of the attack, White House spokesman Sean Spicer said.
Mark Toner, acting spokesperson for the US State Department, said officials were closely following the situation.
"Our hearts go out to those affected. We stand ready to assist in any way the UK authorities would find helpful," he said.
Eyewitnesses report chaotic scenes
People were photographed running in a panic from the scene, while witnesses told CNN they saw the car ram into a crowd.
Robyn Lyon, 34, from Rugby, said he was walking along Westminster Bridge when he saw the vehicle.
"I saw a car, the crunch of car, hitting the curb. I kind of thought the guy had maybe had a heart attack or something," he said.
"The accelerator suddenly picked up so you had that revving. I expected it to hit the bridge wall and then stop, and then turn back carry up the pavement. It hit several people. I jumped out the way into a road."
"I stood in shock and saw carnage around me and the car carrying on up the bridge."
Another witness, Radek Sikorski, said he saw one of the victims bleeding heavily from the head and another lying unconscious.
Richard Poet, 62, who was visiting London from Brighton and his partner were leaving the Tate Britain gallery when he heard "great crashes."
"I thought it was a wrecking ball. It didn't sound like guns or anything."
Craig Meichan, 20, a student from Ormskirk near Liverpool, was on a field trip with around 15 others and had left Parliament just a moment before. He was shaking as he told CNN his part was separated into three groups.
"It sounded like a car backfiring, police began shouting and they started cordoning off the area," he said, adding that he believed his tutor was still locked inside.
"I'm a bit shaken up, you never expect this to happen to you. When it does it's an indescribable feeling."
Chaos in Parliament
As the incident unfolded outside, Leader of the House of Commons, David Lidington, told MPs a serious incident had taken place "within the estate." Proceedings were suspended and MPs were locked inside the main debating chamber.
Lidi
ngton told MPs that it appeared a police officer has been stabbed and that the assailant had been shot by armed police.
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 two 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."
Chris Doyle, Director of Council for Arab-British Understanding, who was at a restaurant at Parliament at the time, described a moment of sudden panic.
"We were in the terrace restaurant when there was a serious of panicked male shouts. Everyone started getting under tables. Under the sofa by me was a small child. Only after several minutes did people get up. It became clear it was the police," he said.
"We still did not know what had happened, if there was a bomb, a gunman a fire. The police directed everyone -- MPs, Lords, cooks, waiters, visitors and schoolchildren -- to the central lobby.
Members of parliament were locked in and asked to keep away from windows.
Hundreds were later evacuated from the Parliament buildings after hours locked inside. Labour MP Barry Sheerman posted an image on Twitter showing lawmakers in the House in the evening with food and drink.
Member of Parliament gives assistance
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.
Eyewitness Hugh Dickinson, a 21-year-old student from High Wycombe, posted a picture of the minister on Twitter, showing the minister bleeding, but saying "he looked to be fine."
