The tram was reported to be traveling at a significantly higher speed than permitted when it derailed south of London.. The tram driver has been arrested on suspicion of manslaughter.
The tram bound for Wimbledon, overturned near Croydon, south of London shortly after 6:00 am local time (0600 UTC) on Wednesday morning.
In addition to the deaths, the London Ambulance Service said eight people had serious or life-threatening injuries. A total of 51 people had been taken to two hospitals.
"After liaison with the coroner, we can confirm that at least seven people have lost their lives as a result of this incident," Deputy Chief Constable Adrian Hanstock said.
A spokesperson for London's fire department said that eight fire engines, five specialist rescue units and more than 70 firefighters attended the scene with heavy cutting equipment to free those trapped under the wreckage. Several people had been freed from the wreckage.
Investigating the cause
Accident investigators said they were probing the cause of the derailment. However, images indicate that the train derailed and tipped over next to an underpass after appearing to come off the rails on a bend. The tram derailed as it was negotiating a sharp, left-hand curve which has a speed limit of 12 mph (19.31 kph.) Trams are not fitted with any safety protection systems for automatic braking if they are going too fast.
"Clearly something has gone catastrophically wrong and we will work tirelessly and quickly with the emergency services, the tram operator First Group and others to establish the cause," Mike Brown, commissioner of Transport for London said.
Local residents said they had heard the early morning crash, followed by shouting. "They started bringing up the casualties, some very seriously injured," a witness told the broadcaster Sky News. "People were carried away on stretchers."
London Mayor Sadiq Khan said "I am in close contact with the emergency services and Transport for London, who are working extremely hard under very difficult circumstances to get the situation under control and treat those who have been injured."
British Prime Minister Theresa May said: "My thoughts and prayers are with all of those who have been affected by this terrible incident that has taken place on the tram in Croydon,"
The crash is believed to be the first fatal incident on a UK tram since 1959.
London's only tram network operates across the south London, linking several suburbs along 17 miles (28 kilometers) of track. It has served an estimated 27 million passengers over the last year.
dm, jm/kl (AP, Reuters)