British police say they're treating incident near Parliament as a 'terrorist incident until we know otherwise'

LONDON, March 22 (Reuters) - A policeman was stabbed inside the perimeter of Britain's parliament building on Wednesday, an assailant was shot outside and several people lay bleeding on a nearby bridge, Reuters witnesses and officials said.

A Reuters photographer saw at least a dozen people injured on Westminster Bridge and photographs showed people lying on the ground, bleeding heavily. The number of casualties was unclear.

The House of Commons, which was in session at the time, was immediately suspended and lawmakers were asked to stay inside.

Photos and tweets from the scene:

31 PHOTOS
Possible terrorist attack outside UK Parliament in London
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Possible terrorist attack outside UK Parliament in London
LONDON, ENGLAND - MARCH 22: An armed police officer stands guard near Westminster Bridge and the Houses of Parliament on March 22, 2017 in London, England. A police officer has been stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard report they have been called to an incident on Westminster Bridge where several people have been injured by a car. (Photo by Jack Taylor/Getty Images)
Terror attack outside U.K. Parliament. 2 men down on road. Man with knife/machete got into Parliament and stabbed p… https://t.co/CxeL2zS6uw
Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain March 22, 2017. REUTERS/Stefan Wermuth
Armed police respond outside Parliament during an incident on Westminster Bridge in London, Britain March 22, 2017. REUTERS/Stefan Wermuth
Armed police have taken the precaution of closing the gates at Buckingham Palace, where the Queen is in residence after today's incident.
LONDON, ENGLAND - MARCH 22: Emergency services vehicles seen on Westminster Bridge on March 22, 2017 in London, England. A police officer was stabbed near to the British Parliament and the alleged assailant shot by armed police. Scotland Yard also reported an incident on Westminster Bridge where one woman has been killed and several people seriously injured by a car. (Photo by Carl Court/Getty Images)
@Reuters photographer at the scene of shootings near House of Parliament in London says at least a dozen injured on Westminster bridge
At least one body seen being covered by a blanket.
People leave after being evacuated from the Houses of Parliament in central London on March 22, 2017 during an emergency incident. Britain's Houses of Parliament were in lockdown on Wednesday after staff said they heard shots fired, triggering a security alert. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
An ambulance is driven through a police cordon outside the Houses of Parliament in central London on March 22, 2017 during an emergency incident. Britain's Houses of Parliament were in lockdown on Wednesday after staff said they heard shots fired, triggering a security alert. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
Police officers surround two individuals on the ground inside the gates of the Houses of Parliament near the entrance, left, in London, U.K. on Wednesday, March 22, 2017. Shots were fired outside Parliament in London, and at least two people are being treated, according to a Bloomberg reporter on the scene. Photographer: Robert Hutton/Bloomberg via Getty Images
Air ambulance just landed at Parliament Square, Westminster, dozens of police cars https://t.co/TZVTO6IzDR
London's air ambulance arrives at the Houses of Parliament in central London on March 22, 2017 during an emergency incident. Britain's Houses of Parliament were in lockdown on Wednesday after staff said they heard shots fired, triggering a security alert. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
A car on Westminster Bridge has just mowed down at least 5 people. https://t.co/tdCR9I0NgJ
We were called at approx 2:40pm to reports of an incident at #Westminster Bridge. Being treated as a firearms incident - police on scene
Incident in #Westminster: We are treating this as a terrorist incident until we know otherwise
Richard tice political campaigner tells me he can see ten bodies on westminster bridge - I can't confirm that from where I am
An armed police officer gestures as he stands guard in Parliament Square, opposite the Houses of Parliament in central London on March 22, 2017 during an emergency incident. British police shot a suspected attacker outside the Houses of Parliament in London on Wednesday after an officer was stabbed in what police said was a 'terrorist' incident. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
People leave after being evacuated from the Houses of Parliament in central London on March 22, 2017 during an emergency incident. Britain's Houses of Parliament were in lockdown on Wednesday after staff said they heard shots fired, triggering a security alert. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
A police officer stops traffic as the Jaguar car of British Prime Minister Theresa May (R) is driven away from the Houses of Parliament in central London on March 22, 2017 during an emergency incident. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
An armed police officer guards inside a police cordon outside the Houses of Parliament in central London on March 22, 2017 during an emergency incident. / AFP PHOTO / Daniel LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
Police officers surround two individuals on the ground inside the gates of the Houses of Parliament near the entrance, left, in London, U.K. on Wednesday, March 22, 2017. Shots were fired outside Parliament in London, and at least two people are being treated, according to a Bloomberg reporter on the scene. Photographer: Robert Hutton/Bloomberg via Getty Images
Police tapes off Parliament Square after reports of loud bangs, in London, Britain, March 22, 2017. REUTERS/Stefan Wermuth
The injured being treated in New Palace Yard at Westminster Palace https://t.co/I7uRyOv2D4
Government source says PM is OK
Police trying to resuscitate one of the Commons attackers. Still seems to be breathing
House of Commons has been suspended following gunshots in Parliament, MPs are leaving the chamber
BREAKING NEWS -- Two people shot outside Houses of Parliament in London
Armed police officers stand guard inside a security cordon, outside of the Houses of Parliament in central London on March 22, 2017 during an emergency incident. Britain's Houses of Parliament were in lockdown on Wednesday after staff said they heard shots fired, triggering a security alert. / AFP PHOTO / DANIEL LEAL-OLIVAS (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)
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Prime Minister Theresa May was safe after the incident, a spokesman for her office said. He declined to say where May was when the attack took place.

The leader of the House, David Lidington, said in the chamber that an assailant who stabbed a policeman had been shot by police.

Amid confusing scenes, it appeared the incident may have unfolded in several locations, including the busy bridge where tourists often congregate to take pictures of Big Ben and other attractions.

The incident took place on the first anniversary of attacks on Brussels in Belgium.

Reuters reporters inside parliament said a large number of armed police, some carrying shields, were pouring into the building.

Britain is on its second-highest level of "severe" meaning an attack by militants is considered highly likely.

In May 2013, two British Islamists stabbed to death soldier Lee Rigby on a street in southeastLondon.

In July 2005, four British Islamists killed 52 commuters and themselves in suicide bombings on the British capital's transport system in what was London's worst peacetime attack.

This is a breaking story. Please refresh for the latest updates.

(Reporting by Toby Melville and UK bureau, writing by Estelle Shirbon, editing by Stephen Addison)

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