According to the still-state-controlled utility, about 23,700 workers in France had joined the industrial action by Thursday evening in one of the biggest strike turnouts at the company in eight years.
More than a third of EDF's workforce in France was on strike on Thursday for the second day in a row in protest against a privatization plan, yet without affecting households, affirmed labor union leaders.
EDF workers are protesting plans steered by the French government to restructure and split the group in two, privatizing its nuclear power generation business set to one side.
The restructuring plan, which is known as "Project Hercule" and was requested by President Emmanuel Macron, will likely involve massive lay-offs, according to the workers on strike.
The project was dubbed a "catastrophe" by CGT union leader Philippe Page Le Merour who denounced the "purely financial purpose, with zero vision for the public service or the industry, and even less for a common good obviously," reported local media.
"The move will tragically end with a medium and long-term rise of electricity bills, since part of the company's activities will be open to the stock market, and share-holders will quickly want to make profits," he added. "Second, this will also be a catastrophe for all the employees of the electricity sector, as the move will pave the way for social dumping."
But unions hope to pile pressure on EDF's management and the government to delay the project, arguing that a split would only weaken the group. The company is to present a final proposal by the end of the year.
"Nobody should forget that the one primarily responsible for EDF's situation today is undoubtedly the state (...) Dismantling EDF cannot be the answer", the unions said in a statement.
The strike was more disruptive than previous stoppages, with four unions representing a majority of France's energy workers joining forces behind the walkout. Previously, the unions have not acted together.
Unions leaders are expected to meet later on Thursday to decide whether further action is needed. The CGT has said it wants another strike on Sept. 24.
After the strike started on Wednesday night, there was a loss of power generation of over 8 percent, and by Thursday midday it had fallen another two percentage points, according to data from EDF and grid operator RTE.
The strike has reduced power generation by some 6 gigawatts, affecting the output only in several nuclear, hydro and gas-fired power plants.
Power station outages will not knock out the grid or hit households, though cuts in power output are costly for EDF, as it has to import any shortfall.
EDF operates all 58 French nuclear reactors, which account for around 75 percent of the country's electricity needs.