LONDON ATTACK: Policeman among 5 dead as terrorist strikes outside UK Parliament

An 'Asian man in his 40s with eight-inch long knife' was reportedly shot dead after he "mowed down" at least a dozen pedestrians by a car and stabbed a policeman close to Britain's Houses of Parliament -- also called Palace of Westminster -- in London on Wednesday in what police said they were treating as a terrorist incident.

Witnesses said a vehicle struck several people on Westminster Bridge, and photos showed a car plowed into railings. According to reports, three people, including the policeman, have died in the attack. 

Although exact details are yet to emerge, it is believed that a man armed with a knife attempted to force his way past a security check-point at the front of Parliament, and attacked a police officer. The attacker was then shot by police.

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.

Numerous witnesses reported hearing gunfire near Portcullis House, an office building housing lawmakers and members of their staff. The Metropolitan Police in London confirmed that officers had been summoned at about 2.40 pm to Westminster Bridge over reports of "a firearms incident."

Police are asking people to avoid the following areas: Parliament Square, Whitehall, Westminster Bridge, Lambeth Bridge, Victoria Street up to the junction with Broadway and the Victoria Embankment up to Embankment tube.

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

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