MPs, journalists, political staff and school children were moved to Westminster Hall to allow police to sweep through buildings in the hunt for another assailant.
Hundreds of people massed 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 were held for up to 90 minutes as possible witnesses. Officers are checking if anyone has photos and videos from the scene, as footage may contain vital evidence.
Police also checked whether anyone needs medical assistance.
London terror attack scene on Westminster Bridge
Mr Heffer added: "Westminster Hall is filling up considerably. There are at least 300 people here including some MPs.
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"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.
Emergency services at the scene of the London terror attack
In a second police press briefing, Mark Rowley, acting deputy commissioner of London Metropolitan Police, confirmed searches were being carried out across the Palaces of Westminster as a precaution.
He said: "We are confident there is just one suspect but as a precaution we are locking down surrounding areas to check for other assailants.
"We ask the public to be vigilant on their way home. You will see more armed police armed on the streets."