The bus swerved to avoid a lorry carrying railway lines
Three people were seriously injured when a double-decker bus crashed on to its side after colliding with a lorry carrying rail tracks in London.
Fire crews had to cut three passengers out of the bus after the metal tracks smashed through its windows in Battersea at about 0830 GMT.
The driver of the C3 bus was among those seriously hurt. Another 14 people were treated for minor injuries.
The lorry driver was arrested on suspicion of careless driving.
Police have released the 48-year-old man on bail until March.
About 30 passengers were on the bus when it crashed at the junction of Plough Road and St John's Hill.
Eyewitnesses said the sections of railway track were protruding over the front of the lorry as it emerged from a junction on to the main road.
Investment banker Rory Withfield, 31, who was on board, said: "I was standing near the front of the bus when I saw the truck approaching.
"It was carrying metal pylons which struck the bus and were ripping the side of it."
He said the driver appeared disorientated and the bus then mounted the pavement and toppled over.
Describing the moment the vehicle crashed on its side, Mr Withfield said: "It was leaning over and then it seemed to fall in slow motion.
"People were screaming and everyone's bags were falling down one side. After it fell over everything was quiet as everyone had realised what had happened."
He continued: "There were one or two people trying to help the rest. The driver seemed unconscious but he came to. We were stuck in there until the emergency services arrived."
Billy Song, 29, an operations manager who lives nearby, saw the crash.
He said: "I heard a bang and saw the bus knocked down. Metal poles were all over the ground."
Mr Song added: "People ran to give some help and I called 999.
"I saw a passenger on the top deck very clearly and he looked injured."
A London Ambulance Service spokesman said: "Fifteen patients were treated at the scene and transported to hospital.
"Three patients were taken as a priority - a 27-year-old woman and a 30-year-old to St George's Hospital, and a 45-year-old man to Chelsea and Westminster.
"Twelve patients were transported to hospital with minor injuries - six to Chelsea and Westminster, four to St Thomas' and two to Charing Cross."
Two people were expected to remain in hospital on Wednesday night, both of them in a stable condition.
Insp Dave Toms-Sheridan, of the Metropolitan Police's collision unit, called the crash "dramatic".
He told the BBC: "The rails from the lorry have hit the side of the bus with such force, it has sent the bus on to its side.
"Anybody sitting on that bus this morning would have been in a very frightening situation - we are fortunate injuries aren't far more serious."
Plough Road has now reopened fully to traffic.
Transport for London has promised "a full investigation".