Moscow's Deputy Mayor Pyotr Biryukov told reporters that at least 149 people had been injured, 106 of whom were hospitalized.
In an announcement on the city's public transit website, officials confirmed that the derailment occurred at about 8:35 a.m. local time between the Slavyansky Bulvar and Park Pobedy metro stations on the Blue Line.
At least 200 people were evacuated from the tunnel, and TV cameras early Tuesday morning showed dozens of the severely injured being carried away on stretchers. Some victims were airlifted to the hospital.
Initial reports suggest a power surge is to blame for the deadly accident.
"The train slowed down abruptly, the lights went off, and then there was a spark of fire and smoke. We were blocked in," described a passenger, who escaped unscathed, told Russian TV.
Park Pobedy, the closest station to the accident, is one of the deepest places in the system, making rescue efforts particularly difficult.