It was unclear what caused the train to derail. Lines of inquiry included a fault in one of the cars or the sinking of the roadbed, according to Vladimir Markin, spokesman for Russia's top investigative body. He said other officials who said earlier that a power surge triggered an alarm, causing the train to stop abruptly, were incorrect.
Of the 136 people hospitalized, at least 42 were in grave condition, health officials said. One citizen of China and one citizen of Tajikistan were among those killed, Russian news agencies quoted city officials as saying.
Over 1,100 people were evacuated from the train, which was stuck between two stations, in a rescue operation that ended more than 12 hours after the accident. One woman taken from the scene died at a Moscow hospital.
In video released by the Emergency Situations Ministry, several wrecked train cars looked almost coiled, occupying the entire width of the tunnel. Workers were trying to force open the mangled doors of one car to retrieve bodies. Photos posted on social media sites showed passengers walking along the tracks in the dimly lit tunnel.
Russian officials rushed to open an investigation into the accident. President Vladimir Putin, who is traveling in Brazil, demanded a detailed investigation into the "reasons for the event" and asked the country's top investigators to open a criminal case, his spokesman told Russian news agencies.
Mayor Sergei Sobyanin also told reporters that unnamed officials will not only be fired but also charged with crimes, though he would not say what charges they might face. Prime Minister Dmitry Medvedev offered his condolences for the victims and their families.
Dozens of injured people were carried out of the station on stretchers. Paramedics carried one woman covered with a blanket to the lawn by the Triumphal Arch, which commemorates Russia's victory over Napoleon, and put her on a helicopter ambulance.
Several survivors sat on the sidewalk near the station's entrance in an apparent state of shock, drinking water supplied by authorities on a hot summer day.
Witnesses told Rossiya 24 television that they felt a sharp impact that knocked them off their feet and hurled them across the train cars.
"Everybody spun to one side," said one witness. "There was a sharp brake, and smoke was in the air. People couldn't leave for a long time. The way was blocked."
While technical glitches are regular occurrences in the Moscow Metro, the subway hasn't seen deadly accidents in decades.
Terrorism is another matter. More than 100 people have been killed in bombings on Moscow's subway trains or near stations since 2000, including two bomb blasts on the same day in 2010 that killed a total of 40 people.
Laura Mills in Moscow contributed reporting.