Five people were killed and more than 50 injured Wednesday when a London tram derailed and tipped on its side, police said, while the driver has been arrested.

The incident happened at around 6:10am (0610 GMT) at a junction outside Croydon in south London. Police have declared a major incident at the scene.

"At present, we can confirm five people have sadly died following this incident," the British Transport Police said in a statement.

"One person has been arrested in connection with the incident and is currently in police custody."

Investigations were under way as to what caused the two-car tram to derail.

The London Ambulance Service said 51 patients had been taken to two local hospitals. Twenty-two ambulance crews were dispatched.

"We have treated a number of patients at the scene, mainly for minor injuries such as cuts and bruises," said deputy operations director Peter McKenna.

"However, very sadly some people have died and others have suffered serious injuries."

One of the hospitals said four of the 20 patients admitted were in a serious condition.

The other said it was assessing and treating patients from the derailment and urged people with minor ailments to go elsewhere.

London Fire Brigade said it had been working to free several trapped people.

London's only tram system, which opened in 2000, links several south London suburbs along 17 miles (28 kilometres) of track.

More than 27 million passengers used the service in 2015-16.

London Mayor Sadiq Khan said the emergency services were "working extremely hard to get the situation under control and treat those who have been injured".