UK Parliament is in lockdown.
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US President Donald Trump "has been briefed on the situation" in London and is monitoring events, his press secretary Sean Spicer tweeted.
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Staff and visitors at St Thomas' and Evelina London hospitals, across the Thames from Parliament, are being advised to remain on site during the current security incident".
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Scotland Yard said in a statement: "Police are asking people to avoid the following areas: Parliament Square; Whitehall; Westminster Bridge; Lambeth Bridge; Victoria Street up to the junction with Broadway and the Victoria Embankment up to Embankment tube.
"This is to allow the emergency services to deal with the ongoing incident.
"Officers - including firearms officers - remain on the scene and we are treating this as a terrorist incident until we know otherwise. "We thank the public for their assistance."
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MPs, who were at the Commons today to grill May at Prime Minister's Questions, reveal their ordeal:
https://mobile.twitter.com/TracyBrabin/status/844573437406445568
https://mobile.twitter.com/Vernon_Coaker/status/844575834883182592?
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Former education secretary Nicky Morgan told the Press Association: "The Division Bell had just gone and MPs were making their way to the House of Commons chamber. I was walking from Portcullis House along the path of Old Palace Yard when suddenly shots rang out.
"It takes a moment to realise that is actually gunfire and at that point people were yelling get down get back"
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People mowed down on Westminster Bridge:
Picture from inside parliament:
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SNP MP Pete Wishart says MPs were evacuated into New Scotland Yard:
https://mobile.twitter.com/PeteWishart/status/844578127066140672
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12 people are thought to have been mown down on Westminster Bridge