Power generation at EDF's 1,300 MW Nogent 1 nuclear power plant was cut by 735 MW, while at the 900 MW Gravelines 1 reactor, output was down 37 per cent.
The RTE said generation at the 585 MW Bouchain 7 gas-fired plant was reduced by 285 MW.
The grid operator did not give reasons for the strike, but France's hard-left trade union CGT has called for a nationwide strike at EDF on Nov 29 to protest over stalled wage negotiations and a possible government-led restructuring of the company.
The union has called for another nationwide protest on December 3.