Seven people have died and dozens more have been injured after a tram overturned in London.
People were trapped inside and more than 50 were taken to hospital after the derailment in Croydon just after 06:00 GMT.
The tram driver, 42, from Beckenham, has been arrested on suspicion of manslaughter.
British Transport Police said they were investigating whether he fell asleep.
The Rail Accident Investigation Branch (RAIB) said the tram was travelling at a "significantly higher speed than is permitted".
The police have set up a number - 0800 0560154 - for friends and family to call for information.
Deputy Chief Constable Adrian Hanstock said: "We can confirm that at least seven people have lost their lives as a result of this incident.
"Our officers will continue to work tirelessly throughout the evening to formally identify them and provide care and support for their families."
London Ambulance Service said eight people have serious or life-threatening injuries, while a total of 51 had been taken to two hospitals.
Robin Smith, Assistant Chief Constable of the British Transport Police, said they were investigating whether the driver fell asleep, alongside "a number of factors".
The RAIB said the Wimbledon-bound tram derailed as it was negotiating a "sharp, left-hand curve" which has a speed limit of 12 mph.
Trams are not fitted with any safety protection systems that apply the brakes automatically if they are going too fast, according to the Office of Rail and Road.
The overturned tram remains on its side next to an underpass and appears to have derailed where the track branches.
Martin Bamford, 30, from Croydon, was on the tram.
He said "everyone just literally went flying", adding that people were screaming and there was "blood everywhere".
Speaking outside Croydon University Hospital, where he is being treated for fractured or broken ribs, Mr Bamford said: "There was a woman that was on top of me ... I don't think she made it at all. She wasn't responsive. "
Kevin Snow, who was travelling to work on the tram, said he noticed it did not seem to slow down at the bend and "the next thing I knew we were on our side".
The carpet fitter, who suffered a badly bruised shoulder, said the carriages slid for eight to ten seconds before coming to a halt.
Kudirat Okesola said her husband was travelling on the tram and suffered a "massive" cut on his face.
She said some people trapped underneath the tram were calling for help.
"People were screaming. People were crying," she said.
Andy Smith said he was waiting at a nearby bus stop when he heard "what sounded like a screeching noise, then a bang".
He said: "[I looked] down the track and I saw carnage. There was a lot of screaming, panicking and commotion. It was a macabre scene."
Croydon Central MP Gavin Barwell said his friend was on the tram.
"He's been injured, he's in hospital, he's going to be ok. But it's been a very shocking day for the whole community here.
"Thousands of people use these every day to get to work, to get to school and they're one of the best things about Croydon; we've got this fantastic modern environmentally-friendly transport system and it's had a great safety record.
"This has come as a great shock."
St George's Hospital in Tooting said it was treating 20 people, four of whom were seriously injured and 16 who were walking wounded.
Clinical director Dr Phil Moss said three were having surgery and could be kept in for "several days or even weeks".
Croydon University Hospital's medical director Dr Nnenna Osuji said 31 patients were brought in by ambulance, while seven others arrived on foot.
Police set up a cordon around the crash site while investigations continue.
Mike Brown, commissioner of Transport for London, said: "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."
The crash is believed to be the first in the UK involving fatalities on board a tram since 1959.
On that occasion two women passengers and the driver died after a tram caught fire in Shettleston Road, Glasgow, following a collision with a lorry.
Prime Minister Theresa May offered her thoughts and prayers for the Croydon victims, while London mayor Sadiq Khan visited the crash site.
A book of condolence has opened at Croydon Town Hall.
At the scene: BBC London's transport correspondent Tom Edwards
It's a very sombre mood in this area.
What struck me is trams, in particular, are very tightly knit to their communities. Everyone uses them so this has hit this community very, very hard.
I saw a lady walking down the street making a sign of the cross.
This is a very dark day for London transport.
- London's only tram network operates from Wimbledon to Beckenham Junction, Elmers End and New Addington, via Croydon
- It is run by Tram Operations Limited, a subsidiary of First Group
- Transport for London is responsible for tram frequency, overall performance, maintenance and improvement work
- The network 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
- Until 1951, trams in Croydon ran along the A23 before they were shut down to make space for more road traffic