Update: 6.15pm At least seven people were killed when a tram sped round a sharp bend and derailed during the morning rush-hour.
Investigators said the vehicle was travelling at a "significantly higher speed than is permitted", and are probing whether the driver, who has been arrested, may have fallen asleep.
Scenes on board have been described as "total carnage" and "like something out of a film" as the two-carriage tram tipped onto its side next to an underpass near the Sandilands stop in Croydon, south London.
The 42-year-old driver, from Beckenham, is being held on suspicion of manslaughter and is currently in police custody, British Transport Police said.
Earlier: A tram which derailed killing at least five people was travelling at a "significantly higher speed than is permitted", accident investigators said.
The driver of the south London tram has been arrested and London mayor Sadiq Khan has warned that the death toll is likely to rise even further.
Sources at the scene said eight people were feared dead after the tram tipped on its side next to an underpass near the Sandilands stop in Croydon.
More than 50 people were injured, some seriously.
Martin Bamford, 30, from Croydon, was on the tram and recalled it "speeding up", adding: "Everyone just literally went flying."
He said people were screaming and there was "blood everywhere", describing the scene as "like something out of a film".
Speaking outside Croydon University Hospital, where he is being treated for fractured or broken ribs, Mr Bamford said: "It was just terrifying."
Asked about what he witnessed as the incident unfolded, he said: "There was a woman that was on top of me ... I don't think she made it at all. She wasn't responsive. There was blood everywhere."
Asked about the driver, he said: "I asked him if he was okay. He said 'yeah'. I said to him 'what happened?' He said he thinks he blacked out."
Mr Bamford, who described the scene as "total carnage", said injuries appeared to range from broken legs to head injuries.
He said the hospital was "manic" when he arrived but praised the staff for how they dealt with the situation.
The Rail Accident Investigation Branch said the tram derailed as it was negotiating a "sharp, left-hand curve" with a speed limit of 12 mph.
A spokesman said: "Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted."