The evacuation of passengers caught in the metro accident on Tuesday is still under way, a Russian interior ministry official said.
Twenty people were killed and up to 120 injured when a Moscow underground train derailed between two stations during the morning rush hour, Russia's Itar-Tass news agency cited the Emergencies Ministry as saying.
Russia's investigative committee said it was looking into the causes of the accident. There seemed to be no suggestion of foul play.
"Number of casualties is being updated, the investigation department of the Moscow Investigative Committee has launched a criminal investigation," said Interior Ministry official Roman Syomushkin to media outside Park Pobedy metro station.
"Several possible causes of the accident are being investigated, the main one is violation of transport safety regulations," Syomushkin added.
Injured passengers were carried on stretchers, bloodied and bandaged, out of metro stations and helicopters ferried the most seriously hurt to hospital. Passengers looked stunned or were crying after being helped to the surface by emergency services.
The evacuation of people trapped underground was still under way, said Deputy Head of Moscow Metro Police, Nikolai Savchenko.
"Our task at the moment is to complete the evacuation of those suffered (in accident), to calm down passengers who are still near the station."
Investigators said a power surge caused the train to stall and several cars to come off the rails between the Slaviansky Boulevard and Park Pobedy stations.
A city transport services spokesman told news agency Interfax that all passengers had been evacuated from the affected stations by midday, dismissing reports that some passengers were still trapped in the underground tunnel.
The Moscow metro is the world's busiest, with as many as 9 million people on week days riding a system that is widely recognized for its reliability.
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.
Islamist militants have previously carried out deadly attacks in Moscow, including twin suicide bombings that killed 40 people on the subway in 2010.
REUTERS