Twenty-one people have died after a train derailed in Moscow's packed metro during rush hour in the worst accident ever to hit one of the world's busiest subways.
Russian television described scenes of chaos and panic on the capital's famed system on Tuesday, saying passengers fell like dominoes when the train travelling at 70km/h braked abruptly and three carriages derailed and crumpled.
Rescue teams were still combing through the mangled metal carriages hours later in an attempt to extricate several bodies.
"I thought it was the end," one surviving passenger said on television. "We were trapped and only got out through a miracle."
President Vladimir Putin, who is on a trip to Brazil, ordered a criminal probe into the tragedy that put a huge strain on the city of some 12 million and snarled traffic on its notoriously clogged roads amid a heatwave.
The Investigative Committee said it was looking at a number of possible causes including a mechanical flaw in a carriage and a power failure.
A terror attack has been ruled out, the committee said.
Sirens wailed as dozens of ambulances rushed to help treat the wounded and helicopters buzzed overhead to evacuate those with serious injuries, AFP journalists said.
Health Minister Veronika Skvortsova told Russian agencies that 21 people had died.
More than 160 people were hospitalised, including 42 still in intensive care, the head of Moscow's health department Georgy Golukhov told journalists.
At least two foreign nationals - a Tajik and a Chinese - were among the dead, Golukhov said.
Paramedics, a police officer and a volunteer carry an injured man out from a subway station after a rush-hour subway train derailment in Moscow. Photo / AP
"This is a huge catastrophe for us," deputy prime minister Olga Golodets said in televised remarks.
The head of the Russian Orthodox Church, Patriarch Kirill, led a prayer to honour the victims, while city hall called for a day of mourning on Wednesday.
The accident raised calls for urgent improvements to the ornate but overcrowded metro, which first opened in 1935 under Stalin.
Moscow's ornate metro is considered one of the world's busiest and carries some nine million people every day.