Firefighters had to free passengers trapped on a double-decker bus which overturned in the morning rush-hour after a collision with a lorry.
The driver of the C3 bus was one of five people seriously hurt after the incident in Clapham, south London.
Police said the male driver of the lorry was arrested and is in custody at a south London police station.
Three of the passengers were cut free by firefighters from Battersea and Wandsworth.
Police said that as well as the four passengers and the driver who had been seriously injured, a further 13 needed hospital treatment for minor injuries.
London Ambulance Service said a 27-year-old woman and 30-year old man were taken "as a priority" to St George's Hospital in Tooting, south London, while another priority case - a 30-year-old man - was taken to Chelsea and Westminster Hospital.
Others were taken to Chelsea and Westminster and to St Thomas' and Charing Cross hospitals.
The incident, with about 25 passengers on the bus, happened at the junction of Plough Road and St John's Hill. The C3, run by the Abellio company, operates between Earl's Court and Clapham Junction.
Transport for London said there would be a full investigation into the incident.
