"The activists will send a message to the Greek government that it cannot ignore," the organisation said of the stunt at a facility near the northern town of Kozani.
The 10 activists climbed a cooling tower at a plant that belongs to Public Power Corporation (PPC), the country's main electricity provider.
"Let's Solarize Greece," read a banner carried by one of the activists.
"We are determined to stay here until we have completed our mission." said Takis Grigoriou, energy supervisor for Greece at Greenpeace.
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Yet around half of Greece's electricity production still comes from coal-fired plants, and PPC is Greece's single largest offender in carbon dioxide emissions.
Citing EU environmental agency statistics, Greenpeace said the use of lignite, a form of brown coal used at the Kozani plant, had burdened Greece with between 5.8 and 19 billion euros ( USD 20 billion) in environmental and health costs between 2008 and 2012.
"To meet the target of a temperature rise of no more than 1.5 degree Celsius... We need 100 percent renewable energy by 2050 globally," Grigoriou said.