The London Ambulance Service said "multiple resources" were being sent to the scene.

Will Heaven, managing editor of The Spectator, said on Twitter that he saw "two casualties -- one on pavement, one edge of road" and reported seeing armed police on the bridge.

BBC reporter Holly Jones, who was there at the time of the incident, said she saw a van driven by a man travelling "at about 50 miles an hour".

She said about five people were being treated for injuries after the vehicle mounted the pavement and hit them.

She said she saw a man, who had his shirt off and was in handcuffs, being arrested by police.