The crash happened during the Tuesday morning rush hour when three train cars derailed on the Arbatsko-Pokrovksaya line between Slavyansky Bulvar and Park Pobedy.
"I was riding in the train car. Then a sharp impact happened which took me off my feet," said one passenger. "The lights went out. Everybody fell down."
"Everybody spun on one side. A sharp brake happened. Smoke was in the air. People couldn't leave for a long time. We were blocked," said another passenger.
The emergency ministry said an electrical issue caused the abrupt stop, causing the train to crash into the wall of the tunnel. The city's electricity company said there was no registered outage in that area.
Russia's federal investigative committee had investigators on the scene to find out whether safety violations led to the accident. While fatal accidents in the system are rare, reports of smoke, signaling problems and loss of power have been reported almost monthly for the past year.
Committee spokesman Vladimir Markin reported that the train derailed going approximately 43 miles (70 kilometers) per hour. Moscow's emergency serves said many of the 102 people taken to hospitals are in critical condition.
"We had a tragic event occur today in the metro. An accident occurred and people died," said Russian Prime Minister Dmitry Medvedev.
This is the deadliest incident the Moscow metro system has seen since 2010, when two suicide bombers killed 38 people and injured more than 60 during the morning commute.
Moscow officials said Tuesday's incident seemed to be an accident and not related to any terrorist activity.