Updates on Wednesday 22 March
One woman has died and a number of others have been hurt - including some with "catastrophic" injuries - in the incident on Westminster Bridge, a junior doctor at St Thomas' Hospital has said.
The London Eye and Sealife London operator said it had locked down the attractions with everyone inside for security.
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Labour MP Pat Glass says she was evacuated from Parliament, along with "lots of groups of school children" who were visiting.
She said despite a few "worried faces" the children "seemed to be fine".
"I've never seen so many armed police in my life," she said.
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The Metropolitan Police have warned again sharing photographs and video of those injured in today's Westminster attacks after images emerge on social media.
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Transport for London says Westminster Tube station has also been closed.
Anna O'Neill
BBC Radio London reporter
Eyewitness Kirsten Hurrell told me she heard shots and saw cyclist lying on the pavement.
She said saw smoke or steam coming from vehicle which crashed into the fence.
BBC Radio London is hearing from witnesses. Call Eddie Nestor on 020 7224 2000.
Statement from London Ambulance Service Deputy Director of Operations Pauline Cranmer:
"We were called at 14:40 to Westminster Bridge to reports of an incident, with the first crew arriving within six minutes.
"We have sent a number of resources to the scene including ambulance crews, London's Air Ambulance and our Hazardous Area Response Team.
"We are working closely with other members of the emergency services at the scene, with our priority being to ensure patients receive the medical help they need as quickly as possible.
"We have declared a major incident and our priority is to assess patients and ensure that they are treated and taken to hospital as soon as possible.
"As we are very busy dealing with this incident, we would ask the public to only call us in a genuine emergency."
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Tweets of solidarity following the suspected terror attacks in Westminster.
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Josephine McDermott
BBC News, London
Eye witnesses have reported seeing a car mowing pedestrians down.
Emergency workers attended at the perimeter fence of the Palace of Westminster where this vehicle crashed.
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London Ambulance Service say they have sent "a number of resources to the scene including ambulance crews, London’s Air Ambulance and our Hazardous Area Response Team".
Sam Francis
BBC News, London
Theresa May, the prime minister, is safe according to reports.
The Guardian reports it has seen a video in which she was rushed in to a car 40 yards from the gates outside Parliament where shots were fired minutes after the incident occurred.
She was ushered by at least eight armed undercover police, some with their firearms drawn, into a waiting black vehicle in Speaker’s Court.
Loud bangs can be heard in the background as she is is ushered in to a car, but it is unclear whether the bangs are gunshots.
Guy's and St Thomas' NHS Foundation Trust have tweeted. St Thomas' Hospital is the other side of Westminster Bridge to the Houses of Parliament.
Car crashed into railings, perimeter fence of Palace of Westminster.
This has just been issued by the Met:
"People are asked to avoid 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."
Image source, APBBC News producer tweets: