At least 20 people were killed and more than 150 injured when a Moscow Metro train derailed during rush hour.

Several carriages left the track on the Arbatsko-Pokrovskaya line, also known as Line Three, on Tuesday morning and the rescue operation has so far lasted more than seven hours.

Initial reports said three people were dead and 30 injured but the count has been rising as the emergency operation continues and casualties are recovered from the subway tunnels.

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Alexander Gavrilov, deputy chief of the Moscow emergency services, said rescuers have recovered seven bodies and are working to extract 12 more from two wrecked train cars.

One woman taken from the scene later died in hospital.

Emergency officials said 50 of more than 150 passengers injured are in a serious condition in hospital.

One person was still trapped in a wrecked train car but he was alive while emergency workers tried to free him, according to officials.

Yuri Akimov, from Moscow's emergency services, said around 1,100 passengers were evacuated from a tunnel between Park Pobedy and Slavyansky Bulvar stations, in the west of Moscow.

Several cars went off the track and pictures taken during the rescue operation showed the mangled carriages filling the tunnel.

Officials initially said a power surge had triggered an alarm, which caused the train to stop abruptly and come off the tracks.

But Vladimir Markin, a spokesman for the Investigative Committee, later told reporters later that a power surge was not the issue and investigators were considering a fault in the train cars or a dip in the ground among possible causes.

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Before the derailment was confirmed, people reported seeing smoke on the underground line but authorities denied the claims.

The Mayor of Moscow, Sergei Sobyanin, said officials would not only be fired for any part in the disaster but could also face criminal charges if the investigation finds negligence or wrongdoing.

The Russian capital's airports and public transport systems have been hit by several terrorist attacks over the past two decades but he said terrorism is not being considered as a possible cause for Tuesday's accident.

The Arbatsko-Pokrovskaya line, which stretches for 28 miles, is the longest in Moscow and was the second underground route to open in the Russian capital in 1938.

Park Pobedy is the deepest metro station the system, at 275 feet underground, which made the rescue particularly hard.

The station serves the vast Park Pobedy, where the Second World War museum, close to Moscow's triumphal arch.

“At 8:39am Moscow time [04:39 GMT] on a stretch between stations of Park Pobedy [Victory Park] and Slavyansky Bulvar there was abrupt deceleration of a train,” a press service employee told Russia Today.

More than 60 buses, 40 ambulances and three helicopters were sent to evacuate the affected stations.

Additional reporting by AP