Twenty people died and scores more were wounded after a train derailed in Moscow's packed metro during rush hour on Tuesday 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 city's famed system, saying passengers fell like dominoes when the train travelling at 70 kilometres (over 40 miles) an hour braked abruptly and three carriages derailed and crumpled.

Rescue teams were combing through the mangled metal carriages in an attempt to extricate several bodies.

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.

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 at the scene outside the Park Pobedy metro station in western Moscow.

Nineteen people perished at the scene and another passenger died of her injuries in hospital, a health ministry spokesman said.

In 14 of Moscow's hospitals, doctors fought to save the lives of severely injured passengers.
In all, nearly 130 people were hospitalised and 42 were in a serious condition.

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.

Citing preliminary information, the Investigative Committee said it was looking at a number of possible causes including a mechanical flaw with a carriage and a power failure.

A terror attack has been ruled out, the committee said.

Moscow deputy mayor Pyotr Biryukov said earlier that several people had been trapped in the train, while the authorities said more than 1,000 had been evacuated.

Passengers said smoke quickly spread through the carriages and rescue workers treated them with oxygen.

The accident took place as temperatures outside soared to around 30 degrees Celsius (86 degrees Fahrenheit).

"After the most thorough investigation there will be not only dismissals but also criminal cases against those who are responsible for this tragedy," Moscow mayor Sergei Sobyanin said.

"What happened is one of the most major accidents of recent times."