Two knifemen have reportedly been shot by police after they stormed the Palace of Westminster and stabbed an officer.
Shortly before, a car ploughed into a crowd on nearby Westminster Bridge leaving 12 people injured.
Police have said that they are treating the incidents as terror-related. It is not yet known whether they are linked.
The House of Commons and Lords is in lockdown, with Westminster station closed.
Here is a timeline of what we know so far.
4.00pm: MPs have been evacuated to New Scotland Yard.
The SNP’s Pete Wishard tweeted: “Got out of the building and into New Scotland Yard with other MPs and staff. Incredible response from emergency services.”
The public gallery has also been evacuated.
3.55pm: Reports say that one person has been taken from Old Palace Yard in an ambulance, while another remains covered under a blanket on the cobbles, seemingly dead. It is unknown who the two people are.
Meanwhile police have widened the cordon around Parliament Square.
3.41pm: Donald Trump has been “has been briefed on the situation”, his press secretary Sean Spicer has said.
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
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3.40pm: London Ambulance Service has released a statement calling the situation a “major incident”.
“We were called at 2.40pm 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.”
3.35pm: Police confirm that they are treating the incident as terrorism.
3.19pm: Transport for London (TfL) confirmed that Westminster Tube station has been closed. The Metropolitan police service tweeted: “Officers – including firearms officers – are on scene and dealing with the incident in #Westminster”
3.18pm: Downing Street confirmed that Theresa May is safe. The Prime Minister was reportedly ushered into a silver Jaguar as what sounded like gunfire was heard near Parliament.
3.15pm: Reports started to emerge on Twitter that a car on Westminster Bridge mowed down at least five people. The latest reports suggest that as many as 12 have been injured.
3.09pm: Commons leader David Lidington told MPs that a “police officer has been stabbed” and the “alleged assailant was shot by armed police”.
2.54pm: The Press Association reported: “There was a major security alert at the Palace of Westminster after a man apparently carrying a knife charged through the gates into the front yard of the parliamentary compound.
“Amid shouts and screams, sounds similar to gunfire rang out.
“Two people were seen to be lying within Old Palace Yard, immediately outside Westminster Hall.
“The sitting in the House of Commons was suspended while police officers sealed off the area around the incident.
“Immediately before the incident, at around 2.45pm, a crowd of passers-by was seen running from the direction of Westminster Bridge and around the corner into Parliament Square.”
London terror attack: People injured on Westminster Bridge
2.45pm: Reports emerged of what sounded like shooting at the House of Commons.
2.40pm: Police were called to Westminster Bridge after reports that a car had mowed down at least five people.
A London Met spokesman said: ”Police were called at 14.40 to a reports of an incident on Westminster Bridge.“It’s being treated as a fire arms incident.