Police this afternoon confirmed that five people died when a tram overturned in an underpass.
The driver of a derailed tram, in which dozens of passengers were injured, has been arrested.
Emergency services are still working at the scene near Sandilands tram stop in Croydon, south London, to free people trapped in the two-carriage vehicle.
More than 50 people were injured, some seriously, when the crowded rush-hour tram tipped on to its side next to an underpass near the stop shortly after 6.10am on Wednesday.
British Transport Police confirmed the man had been arrested but did not give any further detail.
London Fire Brigade earlier said three of the five people trapped had been rescued after they sent eight appliances and more than 70 firefighters to the scene in Addiscombe Road.
It is believed to be the first tram crash involving fatalities on board since 1959, when two women passengers and the driver died after a tram caught fire in Shettlestone Road, Glasgow, following a collision with a lorry.
British Transport Police Assistant Chief Constable Robin Smith said: “A number of people have been taken to hospital with injuries and sadly it looks as though there has been some loss of life.
“It is too early for us to confirm numbers but we are working hard to assess the ongoing incident and we are continuing to focus on recovery efforts.”
Sources at the scene said eight people are feared dead.
Accident investigators are probing how the derailment happened.
People living nearby told of hearing a loud noise and seeing the injured being carried away on stretchers.
The tram appeared to have derailed at a point where the track branches.
Croydon resident Hannah Collier, 23, said it was raining heavily at the time of the incident.
She said: “I heard a massive crash at about 6.15am, then heard shouting, then the emergency services arrived.
“They started bringing up the casualties, some very seriously injured. People were carried away on stretchers.”
Adil Salahi, whose property overlooks the track, said the noise he heard was “sudden”.
The 76-year-old said: “It was about 6.10am or 6.15am and because I was praying. I could not move and try to check what it is. I thought it was something in the garage doors. Then I thought it was some lorry.”
Sue Patel, who lives near the station, said: “I heard a noise at around 6 o’clock and I thought maybe it was a car or something. But then I saw there were helicopters.”
She described the sound as “very loud” and a “very big bang”.
Ms Patel, who said she regularly takes the tram line, said: “There’s quite a big bend. You come through the tunnel and there’s quite a sharp bend.”
Liam Lehane, from the London Ambulance Service, described many of those hurt as “walking wounded” but said others suffered serious injuries.
St George’s Hospital in Tooting, south-west London, is treating 20 people.
A spokesman said: “Of these 20 patients, four are seriously injured. All patients are being cared for by our clinical teams.”
The other 31 transferred to hospital were taken to Croydon University Hospital.
Croydon Health Services NHS Trust said: “Our staff are currently triaging and caring for them so we would ask members of the public with more minor routine ailments to consider going to the local walk-in centre or their GP if possible.”
Transport for London said the line is suspended between Reeves Corner and Addington Village/Harrington Road and is likely to remain closed for the rest of the day.
Mike Brown, London’s Transport Commissioner, said: “All of our thoughts are with those who sadly lost their lives in this incident, those who were injured and the families of those affected.
“We are working closely with the emergency services on site and will continue to work with them during the investigation into what happened.”
London’s only tram network operates in the south of the capital, from Wimbledon to Beckenham Junction, Elmers End and New Addington, via Croydon.
More than 27 million passengers used the service in 2015/16.
The system uses a combination of on-street and segregated running for the 17 miles (27km) of track.
Finn Brennan, of the train drivers’ union Aslef, said the incident was “truly dreadful”.
He said: “Lives have been lost and there are many serious injuries. This is a tragedy for all those involved. For the women and men who set out this morning on their journey to work and suffered this awful accident and their families, the grief and pain must be terrible.
“There will be a full investigation into the causes of this incident by the RAIB (Rail Accident Investigation Branch) and others. There should be no rush to judgment or to place blame until the facts are known.
“For now, our thoughts are with all those injured and lost, their families, Tramlink staff and the emergency service and NHS staff working to save lives as they do every day.”
Anyone concerned about loved ones can call the Casualty Bureau on 0800 056 0154.