Dramatic video captures the moment terror first breaks out in the heart of London.

Video shows the moment when panic hit the streets of London during a terrorist incident in Westminster.

Four people were killed, including a police officer and the attacker. At least 20 more were injured.

Footage,  shot by witness Aaron Tsang, shows people running in fear near the Houses of Parliament as gunshots sound.

In the early afternoon several pedestrians were hit by a car on Westminster Bridge.

ROLLING COVERAGE: London terror attack – Click here for everything you need to know.

Police say the car crashed and the man fled armed with a knife and “tried to enter Parliament”.

The attacker stabbed the police officer outside the Houses of Parliament before being shot dead.

Video shot by BBC News shows people fleeing streets nearby to find safety as police cordon off the area.

A car on Westminster Bridge has just mowed down at least 5 people. pic.twitter.com/tdCR9I0NgJ

— Radosław Sikorski (@sikorskiradek) March 22, 2017

Eyewitness Radoslow Sikorski was in a taxi crossing Westminster Bridge at the time of the incident.

He filmed casualties on the bridge and told BBC News he saw “at least five people seriously injured”.

Emergency services were first alerted at 14.40 GMT, after the car ran down people on the bridge.