Emergency services were called at about 6:10 am (2.10 a.m. ET) 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.
“A number of people have been taken to hospital with injuries and sadly we can say there has been some loss of life,” said Assistant Chief Constable Robin Smith from British Transport Police (BTP).
“It is too early for us to confirm numbers but we are working hard to assess the ongoing incident and are continuing to focus on recovery efforts,” he added.
A BTP spokesman later said the tram’s driver had been arrested, but did not specify why, and that five people had been killed.
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.
Local resident Hannah Collier, 23, told local media she had seen very seriously injured casualties being taken away on stretchers.
“I was in bed watching the election when I heard a massive crash at about 6:15 am, which I thought was the wind,” she said. “Then I heard people shouting and then the emergency services arriving.”
Prime Minister Theresa May said her thoughts and prayers were with those affected by “this terrible incident”.
The Croydon tramlink runs from Wimbledon in southwest London to three destinations in southeast London, and splits just after the Sandilands stop.
Wednesday’s incident was the greatest loss of life in a British rail accident since seven people were killed and 70 injured when a high-speed train from London came off the tracks at Potters Bar station north of the capital in 2002.