Eight people are dead and 54 are unaccounted for after an accident at the Sayano-Shushenskaya hydroelectric power station in Siberia.
It is understood that water pipes ruptured at the plant near Sayanogorsk in the Khakassia region, flooding the turbine room.
The components were undergoing repairs when the accident occurred, causing a large portion of the power unit to break off and puncture the ceiling and wall, allowing water to pour in and flood the chamber.
The facility is Russia's largest hydroelectric power station and the fourth largest in the world.
In a statement, the Kremlin said the accident was due to an unspecified 'hydraulic impact' at the plant which forced the shutdown of all 10 of the station's power units.
Emergency situations ministry official Andrei Klyuyev said rescue divers had pulled out one person from a room underneath the plant's main turbine hall, where there was apparently a cave-in and flooding, but said many more could still be trapped.
'At the moment we cannot determine whether these people were down there or managed to get out somewhere but we know that there were that many people on this shift,' Mr Klyuyev said.
The Russian Emergencies Ministry said a water surge was the most likely cause of the accident and it insisted that the body of the dam at the heart of the power plant had not been damaged.
However, some people living close to the hydroelectric station have sought to leave their homes despite official claims that adjacent villages and towns face no danger of flooding.
The mayor of the nearby town of Abakan, speaking to Echo of Moscow radio, said lines had begun forming outside bakeries and gas stations.
Officials said there was no threat of the dam breaking but added it would take 'years' to repair the damaged power units.
RusHydro, owner of the Sayano-Shushenskaya station, confirmed that production at the plant has been halted indefinitely.
The natural resources ministry also said it was concerned by the environmental impact of the accident, saying an oil slick of more than 5km is spreading along the Yenisei River.
Russia's financial regulators have ordered the suspension of trading on both Moscow stock exchanges in shares of RusHydro.