Londoners were told to stay away from a swathe of central London after a terror attack in Westminster today.

Medical crews were seen treating patients on Westminster Bridge after a 4x4 vehicle mowed down pedestrians and a police officer was stabbed near Parliament.

At least four shots were reportedly heard after the vehicle ploughed into pedestrians on Westminster Bridge. A police officer was then stabbed.

The Met warned Londoners 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.

Attack at Parliament, March 22, 2017 36 show all Attack at Parliament, March 22, 2017 1/36 A policeman points a gun at a man on the floor as emergency services attend the scene outside the Palace of Westminster, London Stefan Rousseau/PA 2/36 Emergency services at the scene outside the Palace of Westminste PA 3/36 The scene of the incident @Lukesteele4 4/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 5/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 6/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 7/36 The flag above the Houses of Parliament in London flies at half mast the day after a terrorist attack Jeremy Selwyn 8/36 A forensics tent on Westminster Bridge seen from Victoria Embankment in London the day after a terrorist attack Jonathan Brady/PA 9/36 Police officers walk accross Westminster Bridge the morning after an attack by a man driving a car and weilding a knife Darren Staples/Reuters 10/36 Emergency services at the scene outside the Palace of Westminster Stefan Rousseau/PA 11/36 A knife on the cobbles at the scene outside the Palace of Westminster, London Stefan Rousseau/PA 12/36 Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London Stefan Rousseau/PA 13/36 A man lies injured after a shooting incident on Westminster Bridge in London Toby Melville/Reuters 14/36 Injured people are assisted after an incident on Westminster Bridge Toby Melville/Reuters 15/36 Police secure the area on the south side of Westminster Bridge close to the Houses of Parliament in London Matt Dunham/AP 16/36 An armed police officer runs accross the road during an incident on Westminster Bridge in London Toby Melville/Reuters 17/36 A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament Carl Court/Getty Images 18/36 A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament Carl Court/Getty Images 19/36 The scene outside the Houses of Parliament 20/36 The area around Westminster was put on immediate lockdown BBC 21/36 Police outside the Palace of Westminster, London Victoria Jones/PA 22/36 Westminster Bridge 23/36 Theresa May during Prime Minister's Questions before the incident PA 24/36 A police officer stops traffic as the Jagaur car of British Prime Minister Theresa May is driven away Daniel Leal-Olivas/AFP/Getty Images 25/36 A image of armed police inside Parliament @BarryGardiner 26/36 Emergency services close to the Palace of Westminster, London Yui Mok/PA 27/36 Police close to the Palace of Westminster, London Victoria Jones/PA Wire 28/36 Police close to the Palace of Westminster, London Yui Mok/PA 29/36 An Air Ambulance outside the Palace of Westminster Victoria Jones/PA 30/36 Emergency personnel close to the Palace of Westminster Yui Mok/PA 31/36 Police forensic officers on Westminster Bridge, close to the Palace of Westminster Dominic Lipinski/PA 32/36 People remain in pods on the London Eye after it was stopped Jonathan Brady/PA 33/36 Mark Rowley, Assistant Commissioner for Specialist Operations in the Metropolitan Police, speaking outside Scotland Yard in London Victoria Jones/PA 34/36 Westminster Bridge BBC 35/36 Emergency services make their way down the river Jack Taylor/Getty Images 36/36 Members of the public wait at locked doors outside St Thomas' hospital Carl Court/Getty Images 1/36 A policeman points a gun at a man on the floor as emergency services attend the scene outside the Palace of Westminster, London Stefan Rousseau/PA 2/36 Emergency services at the scene outside the Palace of Westminste PA 3/36 The scene of the incident @Lukesteele4 4/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 5/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 6/36 The scene at Westminster bridge this morning, the day after a terrorist attack Jeremy Selwyn 7/36 The flag above the Houses of Parliament in London flies at half mast the day after a terrorist attack Jeremy Selwyn 8/36 A forensics tent on Westminster Bridge seen from Victoria Embankment in London the day after a terrorist attack Jonathan Brady/PA 9/36 Police officers walk accross Westminster Bridge the morning after an attack by a man driving a car and weilding a knife Darren Staples/Reuters 10/36 Emergency services at the scene outside the Palace of Westminster Stefan Rousseau/PA 11/36 A knife on the cobbles at the scene outside the Palace of Westminster, London Stefan Rousseau/PA 12/36 Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London Stefan Rousseau/PA 13/36 A man lies injured after a shooting incident on Westminster Bridge in London Toby Melville/Reuters 14/36 Injured people are assisted after an incident on Westminster Bridge Toby Melville/Reuters 15/36 Police secure the area on the south side of Westminster Bridge close to the Houses of Parliament in London Matt Dunham/AP 16/36 An armed police officer runs accross the road during an incident on Westminster Bridge in London Toby Melville/Reuters 17/36 A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament Carl Court/Getty Images 18/36 A member of the public is treated by emergency services near Westminster Bridge and the Houses of Parliament Carl Court/Getty Images 19/36 The scene outside the Houses of Parliament 20/36 The area around Westminster was put on immediate lockdown BBC 21/36 Police outside the Palace of Westminster, London Victoria Jones/PA 22/36 Westminster Bridge 23/36 Theresa May during Prime Minister's Questions before the incident PA 24/36 A police officer stops traffic as the Jagaur car of British Prime Minister Theresa May is driven away Daniel Leal-Olivas/AFP/Getty Images 25/36 A image of armed police inside Parliament @BarryGardiner 26/36 Emergency services close to the Palace of Westminster, London Yui Mok/PA 27/36 Police close to the Palace of Westminster, London Victoria Jones/PA Wire 28/36 Police close to the Palace of Westminster, London Yui Mok/PA 29/36 An Air Ambulance outside the Palace of Westminster Victoria Jones/PA 30/36 Emergency personnel close to the Palace of Westminster Yui Mok/PA 31/36 Police forensic officers on Westminster Bridge, close to the Palace of Westminster Dominic Lipinski/PA 32/36 People remain in pods on the London Eye after it was stopped Jonathan Brady/PA 33/36 Mark Rowley, Assistant Commissioner for Specialist Operations in the Metropolitan Police, speaking outside Scotland Yard in London Victoria Jones/PA 34/36 Westminster Bridge BBC 35/36 Emergency services make their way down the river Jack Taylor/Getty Images 36/36 Members of the public wait at locked doors outside St Thomas' hospital Carl Court/Getty Images

A Scotland Yard spokesman confirmed the attack was being treated as "a terrorist incident until we know otherwise".

The Prime Minister, who had been in PMQs, was in the division lobby when she was alerted and bundled into a car.

Members of the Met Police's new heavily armed SAS-style counter terrorism unit were deployed to the scene with automatic weapons.

The sitting of the House of Commons was suspended and MPs were told to stay in their chambers.

A number of people reported seeing a man with a knife in a confrontation with police before shots were heard.

Scotland Yard confirmed it was dealing with a "firearms incident" and said officers were called at 2.40pm.

Alan Parry, a Sky Sports presenter who was at the scene, said he heard an “almighty crash” and saw a 4x4 crashed into the railings outside Parliament with smoke coming from the bonnet.

He said: “The guy who was the driver of the 4x4 got out of the car and sprinted away from the scene. That was followed by what sounded like gun shots and all hell was let loose, police descended everywhere.”

The London Ambulance Service also confirmed it was responding to the incident but could not immediately provide further information.

MPs and journalists were told to stay in their offices as the incident unfolded in the street outside. Parliamentary business was suspended.

"Then I walked in as police officers and security start rushing out of the front doors on to the street.

"When I got inside I was wondering what the hell was going on and I saw dozens of panicked people running down the street. The first stream was around 30 people and the second stream was 70 people.

"It looked like they were running for their lives."

Alistair Bunkall described the scene on Westminster Bridge as he stood at Waterloo station.

“I’ve counted at least 15 ambulances or paramedic vehicles. There are a number of unmarked police cars here. Police are putting up cordens. There are paramedics walking up to Westminster Bridge.

“Two ambulances have just left with their sirens perhaps heading to St Thomas’ hospital.”

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