MPs, journalists, political staff and school children have been moved to Westminster Hall to allow police to sweep through buildings in the hunt for another assailant.
It is currently unclear whether police are acting on intelligence or as a precaution.
Hundreds of people are massing in Westminster Hall on the oldest part of the Parliamentary estate.
Greg Heffer, political reporter for Express.co.uk, said: "A police officer came into our press room and told to get out.
London Terror Attack: LIVE PICTURES Wed, March 22, 2017
Two dead, 12 injured as knifeman mows into crowd in Westminster
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
"They are searching the rooms, an MP speaking to journalists said police were looking for any unaccounted for suspect.
"The press rooms have been cleared and the same procedure is being carried out in other buildings."
All those in Parliament are being held for up to 90 minutes as possible witnesses. They will check who has photos and videos from the scene, as footage may contain vital evidence.
Police are also checking whether anyone needs medical assistance.
London terror attack scene on Westminster Bridge
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Mr Heffer added: "Westminster Hall is filling up considerably now. There are at least 300 people here including some MPs.
"Armed police are walking around, but that is standard practice in Parliament these days."
A suspected terrorist mowed down a crowd of people on Westminster Bridge just before 3pm today, on the anniversary of the Brussels terror attack.
He then tried to storm Parliament, but was stopped by a brave police officer, who has died in the line of duty.
At least 20 people are injured, and four people have died.
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