UK Parliament attack: Five things we know so far

Air ambulance in London

Four people were killed and 20 injured after an attack by an individual at the British Parliament on Wednesday, in what police said they were treating as a 'terrorist incident'. It appeared the incident may have unfolded in several locations, but details are still emerging.

Here's what we know so far:

1.A man in a car mowed down several pedestrians as he drove across Westminster Bridge, where tourists gather to take pictures of Big Ben. After crashing the vehicles into the railings, he ran into the Palace of Westminster - the British Parliament building - and stabbed a police officer.

2. MPs said they had heard three or four gunshots and staff inside Parliament were told to stay inside their offices. Prime Minister Theresa May was confirmed as safe.

3.  Commons Leader David Lidington told MPs the "alleged assailant was shot by armed police". Photographs showed people lying on the ground, some of them bleeding heavily and one apparently under a bus. A woman was pulled alive, but with serious injuries, from the river Thames.

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

5. The incident has taken place exactly a year after the terrorist attack at the airport and a metro station in the Belgian capital of Brussels that left 35 people dead and more than 300 injured.

Recent terror attacks in London:

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.