French power workers have cut around 3% of state-owned energy group Electricite de France's (EdF) generation capacity, according to union leaders.
Power workers plan to continue their strike action
The action is timed to coincide with a vote by the French Senate to transform Edf and its sister firm Gaz de France (GdF) into limited liability companies.
Unions believe the move will eventually lead to the privatisation of the firms.
The CGT union said the cuts, which amount to 2,700 megawatts of power, would continue until 9pm (1900 GMT).
CGT spokesman Maurice Marion said nuclear production at EdF's power stations in Cattenom, Blayais and Chinon had been affected by the cuts.
Meanwhile, workers are planning to demonstrate outside the Senate - which is France's upper house of parliament - later on Thursday.
Unions believe plans to change the status of EdF and GdF would eventually lead to widespread redundancies, as well as the erosion of worker benefits, retirement rights and job security.
EdF workers achieved their biggest output cuts on 15 June, when they shut down around 15% of the company's generation capacity.
Previous strikes have seen energy supplies cut to Spain, the Eiffel Tower and the homes of senior French politicians, including the country home of Prime Minister Jean-Pierre Raffarin.
Workers have also pledged to continue their so-called Robin Hood operations, which include reconnecting the homes of customers who have not paid their bills and supplying hospitals, charities and social security buildings with power for free.
The most recent wave of strikes, at the beginning of July, marked the official opening up of the electricity and gas market across the European Union, which will see EdF and GdF face increased competition.
Analysts believe EdF's bottom line has already been badly hit by the cuts in production.