The shocked witness said innocent bystanders run for their lives as soon as gunshots were heard outside Westminster station and Portcullis House.
The unnamed witness claimed he heard “three shots” while he was fleeing the area during the attack.
He told other bystanders filming in the area what he witnessed as police swooped in to secure the area.
“I heard, ‘Run, Run!’ so that’s what I did,” he said. “We starting running like we had lost our nuts or something – I could have lost my life or something.”
A witness has described the scenes of the London terror attack on Westminster
I heard, ‘Run, Run!’ so that’s what I did
He told the his audience that he had not seen the gunman but heard “three shots” ring out as he made his own escape from the station.
He continued: “I was 10ft away, it was that loud. I was by the cafe by the station.”
More than one person is believed to have died after two serious incidents this afternoon.
Parliament was immediately locked-down as gunshots were heard in the chamber.
The shots are believed to have been armed police taking down two knifemen.
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In an earlier incident, a car ploughed into a crowd on Westminster Bridge leaving 12 injured. It is unknown if the two incidents are linked.
Scotland Yard are calling the incident a “terror attack until we know otherwise”.
Prime Minister Theresa May was in Westminster at the time of the attack and was bundled into a car by plain-clothed police officers. A Government spokesperson said she was safe.
The House of Commons has been suspended for the day, while MPs are held inside the building.
Express.co.uk was alerted to the incident by a series of blasts before a group of people were seen running towards Parliament from Westminster Bridge.
A handful of people then entered the parliamentary estate before one appeared to repeatedly attack a police officer with an implement.
A Met Police spokesman said: “Police were called at 14.40 to a report of an incident on Westminster Bridge.
“It’s being treated and a firearms incident. Officers are on the scene and dealing with it.”
London Terror Attack: LIVE IMAGES Wed, March 22, 2017
Two people shot outside Parliament as car mows down 10 people in Westminster
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Emergency services at the scene outside the Palace of Westminster, London, after policeman has been stabbed and his apparent attacker shot by officers in a major security incident at the Houses of Parliament
Other witnesses described seeing a middle-aged man carrying a knife and running.
Quentin Letts, the Daily Mail’s sketchwriter, said he saw a man attack police outside Parliament before being shot.
He said: “I saw a thick-set man in black clothes come through the gates into New Palace Yard, just below Big Ben.”
He said the man was carrying what looked like a stick as he carried out the attack.