At least seven people were killed and more than 50 injured after a Croydon Tramlink tram overturned in London, British Transport Police announced on Wednesday.
Prime Minister Theresa May and London Mayor Sadiq Khan sent condolences to the injured and the families of the dead.
The 42-year-old driver of the tram has been arrested on suspicion of manslaughter and is being questioned amid suggestions it failed to brake as it took a corner at speed.
A statement from St George's Hospital said: "Following an overturned tram incident in Croydon early this morning, 20 people have been brought to St George's Hospital in Tooting for treatment".
Investigators said the vehicle was travelling at a "significantly higher speed than is permitted", and are probing whether the driver, who has been arrested, may have fallen asleep.
"At present, we can confirm five people have, sadly, died following this incident", British Transport Police said in a statement.
The Ministry of Foreign Affairs said The New Zealand High Commission in London was liaising with local authorities to determine whether any New Zealanders were caught up in the tram crash. "He said he thinks he blacked out".
Eyewitnesses described scenes in the wake of the crash as "total carnage" after the two-carriage tram tipped over in heavy rain next to an underpass. I thought, he should be braking in a minute.
He added: "I swear the tram lifted onto one side".
TWITTER@HannahCollier1 INJURED Emergency services could be seen taking people away on stretchers
The death toll was seven last night as more than 50 people were injured.
He said the tram slid for between eight and 10 seconds before coming to a halt.
"The woman next to me, I don't think she made it at all. He was there for me through a lot and I loved him like a brother, I'll never forget him and it is terrible to see such a great person go".
Resident Hannah Collier tweeted that she "heard a massive crash" outside her window and could see "emergency services everywhere".
"We expect to be at the scene for at least the next 24 hours, continuing searches and carrying out forensic examinations in support of the investigation into the circumstances, and in order to provide a report for the Coroner", Hanstock said.
Liam Lehane of the London Ambulance Service described many of those hurt as "walking wounded" but said others suffered serious injuries.
The tram derailed as it was negotiating a sharp, left-hand curve which has a speed limit of 12 miles per hour (19.31 kph.) Trams are not fitted with any safety protection systems for automatic braking if they are going too fast.
Initial findings of the RAIB show the tram came off the tracks as it was negotiating a "sharp, left-hand curve" with a speed limit of 12mph.
Ronaldo caps 'dream year' after Madrid contract extension
You totally killed me boys. "This is the club that is in my heart and I will always be linked to it". I still have 10 years, ' the Portugal worldwide boldly claimed.