Four people have died after a knifeman drove into pedestrians and stabbed a policeman before being shot by armed police.
Scotland Yard's anti-terror officer Mark Rowley has confirmed the police officer and attacker are among those who have been confirmed dead.
The attacker drove a grey Hyundai i40 across Westminster Bridge before crashing it into railings and then running through the gates of the Palace of Westminster where he attacked a police officer.
An air ambulance landed in Parliament Square and Paramedics fought to save the life of the attacker and that of his victim, on the courtyard floor in front of Parliament.
Two large knives could be seen at the scene.
Foreign Office minister Tobias Ellwood was among those on hand to help.
One pedestrian, who was hit by the attacker's car before it reached Parliament, was confirmed dead by a doctor at St Thomas' Hospital.
Others on the bridge suffered 'catastrophic injuries'.
London Ambulance Service said at least 10 patients were treated on Westminster Bridge and a party of schoolchildren from France were among those targeted on the bridge.
One woman who apparently jumped into the River Thames to escape the attacker as he drove across the bridge, was rescued from the river and given urgent medical attention.
"A guy came past my right shoulder with a big knife and just started plunging it into the policeman."
Eyewitnesses described scenes of terror when gunfire sounded from within the Houses of Parliament.
Eyewitness Rick Longley said: "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."
Another witness, 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."
21-year-old Pat McCormack from Washington in Tyne-on-Wear added: "I saw him stabbing the officer in the back of the head and the back of the neck.
"He was running away but then he collapsed."
According to Downing Street, Prime Minister Theresa May was chairing a meeting of the Government's emergency Cobra committee.
She was ushered into a silver Jaguar in the grounds of the palace.
Commander BJ Harrington from the Met Police said there were "a number of casualties" including police officers.
He has said that anti-terrorism measures were being reviewed and urged Londoners to be vigilant.
Surrounding streets are closed to traffic as armed police cleared the area following the incident and Parliament went into lockdown.
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