The crash occurred as the tram negotiated a sharp bend in the track. Fire crews arrived at the scene and were able to free a number of people. The 42-year-old tram driver from Beckenham, was arrested on suspicion of manslaughter and is in police custody.
Shocked passengers told of carnage and blood everywhere as they likened the derailment to a movie scene. Recalling what happened, one of the survivors, a Nigerian man, Taiye Ajibola, who was on his way to work, said the incident occurred immediately they left Lloyds Park.
His wife, Kudirat Okesola, 46, rushed to his side as soon as she heard he was involved in the accident.
"Even my husband was covered with blood," she said.
Ms Okesola said some people were trapped underneath the tram calling for help.
"People were screaming. People were crying," she added.
British Transport Police (BTP) has not yet officially confirmed the identities of the victims. Officers are investigating claims the tram was exceeding permitted speeds and that the driver may have fallen asleep.
The Rail Accident Investigation Branch (RAIB) said the tram, which had been travelling from New Addington to Wimbledon derailed as it was negotiating a "sharp, left-hand curve" which has a speed limit of 12mph. A spokesman said:
"Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted."
A BTP investigator said a number of factors, including whether the driver had fallen asleep or blacked out at the wheel, were being examined as possible causes.
London Ambulance Service said a total of 51 injured casualties taken to two hospitals, with eight having serious or life-threatening injuries. London Mayor Sadiq Khan, who visited the crash site on Wednesday, warned the number of dead may increase.
St George's Hospital in Tooting said it treated four seriously injured victims and 16 walking wounded. Clinical director Dr Phil Moss said three had undergone surgery and could be kept in for several days or even weeks.
Croydon University Hospital's medical director Dr Nnenna Osuji said 31 patients had been brought in by ambulance, while seven others had arrived on foot.
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Source: BBC News