At least seven people were killed and more than 50 injured when a tram overturned on Wednesday morning in south London.
Police said the driver had been arrested, Reuters reports.
Early indications were that the tram was traveling too fast on a sharp bend, investigators said.
Emergency services were called at about 6:10 a.m. to reports that the two-carriage tram had derailed near a stop close to central Croydon, a large suburb to the south of the British capital and a major transport hub.
“We can confirm a total of at least seven people have sadly died at Croydon,” police said in a statement. “Officers continue to work at the scene.”
A police spokesman later said the tram’s driver had been arrested but did not say why.
The independent Rail Accident Investigation Branch said the derailment had happened on a sharp curve with a 12 mph (20 kph) speed limit.
“Initial indications suggest that the tram was travelling at a significantly higher speed than is permitted,” it said in a statement.
A London Ambulance spokesman said 51 others had been taken to hospital, some with serious injuries.
Eight fire engines, five specialist rescue units and more than 70 firefighters attended the scene using heavy cutting equipment to free those trapped.
Television pictures showed the tram lying on its side across the tracks not far from the Sandilands stop near a junction where the line divides.
Police said investigations were ongoing into the cause of the accident which took place in heavy rain.
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