Twenty people died and scores more were hurt after a train derailed in Moscow's packed metro during rush hour on Tuesday in the worst accident 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 braked abruptly and three carriages derailed.

President Vladimir Putin, who is currently on a trip to Brazil, was informed of 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 the most serious injuries, AFP journalists said at the scene outside the deep 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 televised remarks, adding that nearly 130 people were hospitalised.

'Only a miracle'

Rescue teams were working to free about five people believed to be stuck in a train underground, Moscow deputy mayor Pyotr Biryukov told journalists at the scene.

"There are those who've been trapped, they are showing signs of life," he said.

Around half of those who were hospitalised were in a serious condition, authorities said.

"I thought it was the end," one passenger said on television. "We were trapped and only got out through a miracle.

"There were lots of injured. Various injuries: heads, legs."

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

The accident took place as temperatures outside soared to 28 degrees Celsius (82 degrees Fahrenheit).

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

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

Alexei Naryshkin, a presenter on the popular Echo of Moscow radio station, posted a photograph on Twitter of rescue workers carrying a body in a black bag.

"They are laying out the injured. They constantly go down with stretchers. They carry them out. Some are unconscious. Some are moaning with pain," Naryshkin wrote.

'Panic erupted'

Another witness, a young man in a polo shirt, said on television: "I got into the carriage and after about 20 seconds, the light went out and the train was just pulled apart. I was just thrown into the centre of the carriage."

"Panic erupted," he told Life News television.

"We climbed out of the carriages and we saw a blockage, men took hammers and pliers and broke it down and we walked on. The train was smashed, the chassis was just pulled apart."

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.

Television footage showed rescue workers carrying bloodied passengers away on stretchers, while paramedics treated some on nearby grass verges.

Sobbing people, some with bandages around their heads, were shown seated on chairs outside the metro.

Citing preliminary information, the Investigative Committee said the accident happened due to a train braking abruptly because of a false alarm triggered by a supply voltage dip.

A probe has been opened into the accident, the committee's representative, Roman Syomushkin, told reporters.

"This is the most serious man-made disaster in the Moscow underground in its entire history," infrastructure expert Alexei Khazbiyev told AFP.

"It is the most serious accident apart from terrorist attacks."

He slammed authorities for not doing enough to modernise the overcrowded metro system, stressing it could not cope with the current passenger traffic.

"We are paying for poor-quality and dangerous services," 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.

The train derailed between Park Pobedy and Slavyansky Boulevard metro stations on the blue line.

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

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