An inquiry was under way on Wednesday after French trade unionists turned the lights off on a keynote speech by Prime Minister Manuel Valls in protest at planned energy reforms.
Valls was starting a speech to about 700 people in support of his Socialist party's candidate for a by-election on Tuesday night when the lights went out and his microphone turned silent.
The electricity stayed off for about an hour, not just at the venue, but in the whole neighbourhood in Audincourt, eastern France. Valls resumed his speech once the power had come back on.
The mines and energy federation of France's militant CGT union said in a statement it supported the action.
"When we are not listened to, we must make our point differently, so we cut the power, voila," regional CGT head Marc Pautot said on radio France Info.
Electricity distribution group ERDF, part of state-controlled utility EDF, said it was launching an inquiry into the incident.
The CGT union has been behind a number of direct protest actions in France, including the "boss-nappings" that made headlines in recent years.
Valls speech came as an unpopular Socialist government enjoys a resurgence in opinion polls in the aftermath of the Islamists militant attacks in Paris earlier in January, but amid delicate negotiations between bosses and unions over a wide range of pro-growth reforms some union activists see as eroding workers' rights.
France's energy reforms have a wider purpose, aiming to boost the use of renewable energy like solar and wind and reduce the share of nuclear energy in French power production to 50 percent from more than 75 percent now.
Unions say they will increase greenhouse gas emissions, push up electricity prices and increase the risk of blackouts.
The energy reform law is under review in France's upper house of parliament after a first draft was adopted by the lower house late in 2014. The CGT and another union have also called for a one-day strike on Thursday.
The strike is expected to cause only a minor reduction in electric output.
