Five people have died and dozens more have been injured after a tram overturned in south London, police have said.
British Transport Police (BTP) arrested the driver but would not confirm what for.
People were trapped inside the vehicle and more than 50 were taken to hospital after the derailment in Croydon just after 06:00 GMT.
Mayor of London Sadiq Khan said the number of dead “may well increase”.
Investigators from the Rail Accident Investigation Branch are at the scene.
Firefighters were at the scene of a derailed tram near the Sandilands tram stop on Addiscombe Road in Croydon, CR0.
Two people are trapped and more than 50 have been taken to hospital after a tram overturned in south London, the city’s fire brigade has said.
Crews freed a number of people but two remain inside the tram, which derailed in Croydon just after 06:00 GMT.
The Met Police described it as a “serious incident”.
Hannah Collier, who lives nearby, said she heard “a big crash” and saw people being carried away on stretchers.
Eight fire engines and five fire rescue units (FRUs), which contain specialist rescue equipment were are at the scene. Two carriages were derailed.
Fire rescue units were equipped with heavy lifting, winching, cutting and pulling tools, floodlighting, longer duration breathing apparatus, portable generators and other specialised equipment. FRU crews are specially trained and equipped to handle complex rescues, including those from road and rail accidents, water, mud and ice, urban search and rescue incidents such as collapsed buildings, chemical spills and difficult rescues at height.
London Fire Brigade Deputy Assistant Commissioner, Andy Roe, who is in charge of the incident for the Brigade, said: “Firefighters have released a number of people and two people remain trapped. All the emergency services were working closely together to help those involved in this incident.”
The Brigade was called at 0611 and the first fire engine was on the scene within six minutes of being called. The Brigade said firefighters are expected to remain at the scene for a number of hours.
The Met Police, British Transport Police and London Ambulance Service are also at the scene.
- London’s only tram network operates from Wimbledon to Beckenham Junction, Elmers End and New Addington, via Croydon
- It began operation in May 2000 as Croydon Tramlink, becoming the first tram system in London since 1952
- More than 27 million passengers used the service in 2015/16
- The 17 miles (28km) network consists of 39 stops