A group of people were seen running from Westminster Bridge into Parliament Square
From Express.co.uk's offices at Parliament looking across the cobbled New Palace Yard, a group of people were seen running from Westminster Bridge into Parliament Square.
They then ran towards Carriage Gate where MPs and ministers' cars come and go from the parliamentary estate.
Some of the group appeared to be running in panic while one headed straight towards a police officer just inside the parliamentary estate.
Holding a metal implement in their hand, they then struck the officer numerous times.
Another police officer wearing a hi-viz jacket rushed towards the attacker with his baton poised while a further officer rushed to shut Carriage Gate.
As further security staff streamed from parliamentary buildings, two individuals were either being held on the ground or aided inside the estate.
Journalists were ordered away from windows overlooking New Palace Yard by police while being told to remain in offices with the doors shut.
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Conservative MP Tobias Ellwood (left) stands amongst the emergency services at the scene outside the Palace of Westminster, London
Some of the group appeared to be running in panic while one headed straight towards a police officer
Later, an air ambulance landed in Parliament Square along with numerous ambulances and police cars parked in the vicinity.
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I heard some shouting and saw some running out of the corner of my eye
One ambulance entered New Palace Yard.
Both uniformed and non-uniformed armed police stood in groups inside the estate.
Another witness, Don Brind, said he heard shots and then saw two people apparently injured on the ground.
Journalists were ordered away from windows overlooking New Palace Yard by police
Mr Brind, a researcher for MPs and a former Labour party press officer, said: "I heard some shouting and saw some running out of the corner of my eye and then a short time after that there was a shot."
He saw one person on the ground who he assumed was a civilian in dark clothing, with another person he took to be a police officer standing over him, apparently pointing a weapon.
Ten yards away he saw another person in a hi-viz jacket, who he took to be a policeman who "appeared to be alive and conscious and talking to another policeman".