An assailant stabbed a policeman and was shot by police just outside Britain's parliament building in London on Wednesday in what police described as a "terrorist incident".

Amid confusing scenes, it appeared the incident may have unfolded in several locations, including on the nearby Westminster bridge where eyewitnesses said a car had crashed into pedestrians.

Two people died in the incident, according to Sky News, but the total 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.

Officers – including firearms officers – remain on scene in #Westminster. More info as and when verified

— Metropolitan Police (@metpoliceuk) March 22, 2017

The incident took place on the first anniversary of attacks by Islamist militants that killed 32 people in Brussels.

Reuters reporters inside the parliament building heard loud bangs and shortly afterwards saw two people lying on the ground in a courtyard just outside, within the perimeter of the parliamentary estate.

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.

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.

Nearby Westminster underground train station was closed at the request of police.

Photographs by Reuters staffer Toby Melville showed two people lying on Westminster Bridge, bleeding heavily.

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

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

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 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.