A policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament.
The attacker mowed down several pedestrians as he drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings then running through the gates of the Palace of Westminster and stabbing the officer.
Eyewitnesses said he was shot by police as he approached a second officer clutching his knife.
He left a trail of destruction as paramedics tended to his victims on the bridge and at the gate.
Prime Minister Theresa May was seen being ushered into a silver Jaguar in the grounds of the palace as what sounded like gunfire rang out at around 2.45pm.
A Downing Street source confirmed she was "OK".
Sir Jeffrey Donaldson, DUP MP for Lagan Valley tweeted: "Walking into Parliament when attack occurred. Police excellent. Got us onto ground then led to safety. Terrorism will never defeat democracy."
Eyewitness Rick Longley described the attack.
"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.
"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."
Around three shots rang out as the attacker ran through the gates into the front yard of the parliamentary compound.
Two people - thought to be the knifeman and the officer - were then seen being treated on the ground in New Palace Yard amid shouts and screams.
An air ambulance landed in Parliament Square and a regular ambulance came in through the front gates as medics rushed to help the injured people.
Armed police cleared the area around the incident and Parliament Square was closed to traffic while Parliament went into lockdown.
As the sitting in the House of Commons was suspended, Commons Leader David Lidington told MPs: "What I am able to say to the House is there has been a serious incident within the estate.
"It seems that a police officer has been stabbed, that the alleged assailant was shot by armed police.
"An air ambulance is currently attending the scene to remove the casualties."
Witnesses described seeing a middle-aged man carrying a knife and running.
Jayne Wilkinson said: "We were taking photos of Big Ben and we saw all the people running towards us, and then there was an Asian guy in about his 40s carrying a knife about seven or eight inches long.
"And then there were three shots fired, and then we crossed the road and looked over. The man was on the floor with blood.
"He had a lightweight jacket on, dark trousers and a shirt.
"He was running through those gates, towards Parliament, and the police were chasing him."
Her partner David Turner added: "There was a stampede of people running out.
"You saw the people and you thought 'what the hell is going on'."
A witness, who did not want to be named, saw Mrs May and her undercover police detail near the area of Parliament where ministerial cars are parked.
He said there were undercover officers in a 4x4 vehicle in the car park.
"We just heard gunshots inside Parliament," he said.
"It looked like they are taking her away."
Another witness, from South Africa and living in Ireland, said she and two friends went towards the palace gates and saw a man lying on the floor inside.
She said: "I saw a body, they were working on the floor. They were just inside the gates.
"He had one gunshot wound to the right side of his chest. I don't know if he was dead or alive, but people were working on him.
"I saw that needed medical attention and I asked if I could help because I am a doctor, but they said they were fine.
"Then the police moved us back."