As rolling strikes began, 13 power plants were not functioning by the afternoon. The company declared an "amber alert," indicating periodic interruptions to the supply of electricity could be expected at peak usage times.
Workers object to a government plan to privatize 17 percent of its majority stake in the corporation, and the creation of a subsidy to sell about 30 percent of the electrical output to private investors.
The sale of part of the government's portion of the corporation is a demand by the European Union and the International Monetary Fund -- a reform promised in exchange for two financial bailouts totaling 240 billion euros ($325 billion). Greece received the funding in exchange for promises to sell state property and initiate austerity measures.
Workers' unions have contended electricity is a commodity that needs to remain under state control.
Prime Minister Antonis Samaras warned a mobilization order could be used to force workers to return to their jobs.