LONDON – At least seven people were killed and around 50 injured on Wednesday when a London tram came off the tracks and tipped over.

One survivor recounted how a woman landed on top of him, adding that there was "blood everywhere" after the tram appeared to speed up before coming off the rails.

The incident happened on Wednesday before dawn at around 6:10am outside Croydon in south London as it negotiated a bend in the track.

The 42-year-old driver had been arrested on suspicion of manslaughter, British Transport Police said.

"Everyone just literally went flying," said Martin Bamford, 30, from Croydon, who was on the tram and recalled it "speeding up" before it derailed.

"There was a woman that was on top of me... I don’t think she made it at all. She wasn’t responsive."

Asked about the driver, Bamford said: "I asked him if he was okay. He said ’yeah’. I said to him ’what happened?’ He said he thinks he blacked out."

Bamford was speaking outside Croydon University Hospital, where he was being treated for fractured or broken ribs.

Probes were under way to determine what caused the 300-tonne, two-car tram to derail and tip over onto its side.

The Rail Accident Investigations Branch said the tram derailed as it was negotiating a "sharp, left-hand curve" with a speed limit of 19 kilometres per hour.

A spokesman said: "Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted."

Loud bang

Deputy Chief Constable Adrian Hanstock of the British Transport Police (BTP) indicated that the death toll could rise further.

"At least seven people have lost their lives as a result of this incident," he said in a statement.

"Our officers will continue to work tirelessly throughout the evening to formally identify them and provide care and support for their families.

"We expect to be at the scene for at least the next 24 hours, continuing searches and carrying out forensic examinations."

Local residents described hearing a loud bang and seeing the wounded being carried away on stretchers.

The London Ambulance Service said 51 patients had been taken to two local hospitals.

Several people were treated for minor injuries such as cuts and bruises at the scene.

Doctor Phil Moss, the clinical director of the emergency department at St George’s Hospital, said they treated 20 patients.

"The patients who have gone to the operating theatre will certainly be kept overnight if not for several days or even weeks," he said.

London Fire Brigade said it had been working to free several trapped people.

London’s only tram system, which opened in 2000, links several south London suburbs along 28 kilometres of track.

More than 27 million passengers used the service in 2015-16. – AFP