UK Parliament attack: Assailant shot, police say treating it as terrorist incident

London: A policeman was stabbed, an assailant shot and several people injured on Wednesday close to Britain's Houses of Parliament in what police said they were treating as a terrorist incident.

Reuters reporters inside the building heard loud bangs and shortly afterwards a Reuters photographer said he saw at least a dozen people injured on Westminster Bridge, next to parliament.

His photographs showed people lying on the ground, some of them bleeding heavily and one apparently under a bus. The number of casualties was unclear.

"Officers - including firearms officers - remain on the scene and we are treating this as a terrorist incident until we know otherwise," London's Metropolitan Police said in a statement.

An assailant who stabbed a policeman inside the perimeter of Britain's Parliament was shot by police, David Lidington, the leader of the House of Commons, told Parliament.

"The alleged assailant was shot by armed police, an ambulance is currently attending the scene to remove the casualties," he said.

"There are also reports of further violent incidents in the Palace of Westminster but it would be wrong of me to go into further details before confirmation from the police."

Police close the area near Houses of Parliament, London, after sounds similar to gunfire have been heard close to the Houses of Parliament, London. (Photo: AP)

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

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.

An ambulance helicopter landed on Parliament Square, just outside the building.

The BBC said police believed there was a suspect vehicle outside parliament but police did not immediately confirm that report.

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

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

Police secure the area on the south side of Westminster Bridge close to the Houses of Parliament in London. (Photo: AP)

US President Donald Trump told reporters at the White House he had been briefed on events in London but gave no details.

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

Britain is on its second-highest alert 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 southeast London.

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.