A rush-hour subway train derailed Tuesday in Moscow, killing at least 20 people and injuring more than 150, many with serious injuries, the Emergencies Ministry said.
Three cars derailed in the tunnel after a power surge triggered an alarm, which caused the train to stop abruptly.
Russian television described scenes of chaos and panic on the capital city's famed system, saying passengers fell like dominoes when the train braked.
Russia's investigative committee said it was looking into the causes of the accident. It said, however, there was no suspicion of a militant attack, the cause for scores of deaths in Moscow's underground in years past.
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 evacuation of those suffered (in accident), to calm down passengers which are still near the station,” Savchenko said.
More than 100 people had been hospitalized, and at least 50 of the injured are in grave condition, said Moscow health department chief Georgy Golukhov, according to the Itar-TASS news agency.
Rescuers have recovered seven bodies and are working to extract 12 more trapped in two wrecked train cars, Alexander Gavrilov, deputy chief of the Moscow emergency services, told reporters in a televised call.
Gavrilov, of the emergency situations ministry, said outside the Park Pobedy station in west Moscow that more than 1,100 people were evacuated from the train.
Passengers said that smoke quickly spread through the carriages and rescue workers treated them with oxygen.
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.
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, the French news agency AFP reported.
The Moscow metro is the world's busiest, with as many as 9 million people on weekdays riding a system that is widely recognized for its reliability.
The subway cars derailed between the Slaviansky Boulevard and Park Pobedy stations.
Park Pobedy is the deepest metro station in Moscow's subway system - 84 meters (275 feet) deep - which made the rescue particularly hard.
Traffic on between the stations is likely to be suspended for at least two days, Mayor Sergei Sobyanin said.
The station serves the vast Park Pobedy, where the World War II museum is located and which is close to Moscow's triumphal arch.
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.
The accident is believed to be the most serious in the eight-decade history of the city's metro system.
Some information for this report provided by Reuters, AFP and AP.