A transformer exploded during repair works, destroying walls and the ceiling in an engine room where turbines are located and causing the room to flood, the Investigative Committee of the federal prosecutors office said.
The plant's dam was not damaged, and the accident posed no threat to towns downstream, Emergency Situations Minister Sergei Shoigu said.
The accident caused an oil spill, however, and the slick was floating down the Yenisei, the ministry said.
Two of the plant's 10 turbines were destroyed and one was seriously damaged, said Vasily Zubakin, acting chief executive of the plant's owner, RusHydrop.
Shoigu said the repairs would be difficult. "We're probably talking about years rather than months to restore three of the 10 turbines," he said on state-run television.
Electricity supplies from other plants were being routed to areas normally serviced by the Sayano-Shushinskaya station, Shoigu said.
The world's largest aluminum producer, Rusal, was operating as usual, with its smelters being powered from other plants, company spokeswoman Yelena Shuliveistrova said.
Power shortages were reported, however, in the city of Tomsk and the mining area of Kuzbass, state RIA Novosti news agency said.
Aging infrastructure has long been regarded as a key obstacle to Russia's development.
Analysts have warned that Russia needs to boost its power production significantly to meet the growing demand of industrial producers or it would face regular power shortages as soon as several years from now. Monday's accident put these plans in jeopardy.
RusHydro's stocks were down more than 7 percent on the MICEX on Monday morning when the stock exchange suspended trading for its shares.
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