Four people have died including a police officer in a terror attack in London

A vehicle mowing down pedestrians near and suspected terrorist plowed a car into pedestrians on London's Westminster Bridge on Wednesday afternoon, then fatally stabbed a police officer before being shot.

According to the police 40 others were injured in the British Parliament complex on Wednesday, two of whom are also in serious condition.

The 4x4 vehicle slammed into pedestrians on the Westminster Bridge, before crashing into a railing near Parliament.

Parliament was placed on lockdown for several hours and lawmakers were forced to remain in the main debating chamber of the House of Commons as officers rushed to the scene outside.