The attack took place in Kabardino-Balkariya, one of the republics in Russia's volatile Caucasus region where clashes with insurgents are frequent.
A spokesman for the republic's police, Adlan Kakakuyev, said two cars carrying at least six attackers targeted the Baksan plant, killing two guards and wounding three others.
They then detonated explosives in several parts of the 25-megawatt plant before fleeing.
There were no immediate claims of responsibility for the attack.
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