At least 20 people were killed and more than 120 were injured when a Moscow underground train derailed during the morning rush hour in one of the worst accidents on the Russian capital's subway system in years.

President Vladimir Putin, who is away in Brazil, ordered a criminal investigation into the accident, which is likely to raise further questions over Russia's transport safety record. There was no suspicion of a militant attack — the cause of scores of deaths in Moscow's subway system in years past.

Russia Subway Derailment

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, Russia, on Tuesday. (Ivan Sekretarev/Associated Press)

Three carriages derailed on a train travelling between two stations at 70 km per hour at about  8:35 a.m. local time (midnight ET) on the world's busiest subway system.

"It braked very hard. The lights went off and there was lots of smoke," a man, his nose bloodied, told Rossiya-24 television.

"We were trapped and only got out by some miracle. I thought it was the end. Many people were hurt, mostly in the first rail car because the cars ran into each other."

Rescue workers evacuated more than 1,000 people from the area of the accident, the Emergencies Ministry said. Injured passengers were carried on stretchers, bloodied and bandaged, out of both metro stations.

Helicopters ferried the most seriously hurt to hospital and passengers helped to the surface by emergency services looked stunned or were crying.

"There is no one alive left," Moscow deputy mayor Peter Biryukov said. "The cause is not known, the work continues."

Some bodies were recovered from the wreckage but others remained underground, crushed in the accident, he said.

The Investigative Committee put the toll at 20 dead. Russian news agency Itar-Tass quoted Health Ministry representative Oleg Salagai as saying that 129 people were injured, 42 of whom were in a serious condition.

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Moscow is famous for its museum-like metro stations such as the Mayakovskaya above. (Lucy Nicholson/Reuters)

The Investigative Committee, which answers to Putin, said it had opened a criminal case on suspicion of failure to meet safety guidelines but that it had not yet determined the cause of the accident, although no militant involvement was suspected.

A power surge may have caused the train to stall and several cars to come off the rails between the Slaviansky Boulevard and Park Pobedy stations, the Investigative Committee said.

Russia Subway Derailment

The subway train derailed during Moscow's busy rush hour commute. (Ivan Sekretarev/Associated Press)

Russians regularly criticize the country's transport safety record. Recent disasters included the 2011 sinking of a ferry boat that killed 128 and an airplane crash that killed the Lokomotiv Yaroslavl ice hockey team the same year, both of which were blamed on lax safety regulations and technical errors.

On weekdays as many as nine million people use the subway system.

Famed for its high-vaulted halls adorned with Soviet socialist realist art, the underground network has expanded from 13 stations opened in 1935 to 194 stations across the megalopolis today.

Park Pobedy, where the derailment occurred, is Moscow's deepest metro station — 84 metres below the surface — which made the rescue particularly difficult. The station serves the vast park where Russia's World War II museum is located.

Islamist militants have previously carried out deadly attacks in Moscow, including twin suicide bombings that killed 40 people on the subway in 2010.  

With files from The Associated Press