UPDATE: 21 dead after train derails in Moscow metro

A train has derailed during rush hour in Moscow, leaving 21 people dead and more than 160 people in hospital, including 42 in intensive care.

Russian television described scenes of chaos and panic on the capital city's famed system, saying passengers fell like dominoes when the train braked abruptly and three of the carriages derailed.

The train derailed between Park Pobedy and Slavyansky Boulevard metro stations .

Sirens wailed as dozens of ambulances rushed to help treat the wounded and helicopters buzzed overhead to evacuate those with the most serious injuries, AFP journalists said at the scene outside deep Park Pobedy metro station in western Moscow.

Authorities said more than 100 people had been hospitalised, half of them in a serious condition.

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

The accident took place as temperatures outside soared to 28C.

Authorities said it would take 24 hours to deal with the aftermath of the accident.

As news of the accident spread, the hashtag #metro quickly become one of the most popular on Twitter in Russia.

Moscow mayor Sergei Sobyanin rushed to the scene and pledged to do everything possible to help the victims.

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

Opened under Stalin in 1935, Moscow's ornate metro is considered one of the world's busiest and carries some nine million people every day.

Moscow's deepest station, Park Pobedy opened 2003, while Slavyansky Boulevard opened five years later.

In the last serious accident in 1982, eight people died on the Moscow metro when an escalator broke down.