Power generation at one nuclear reactor, EDF's 1,300 MW Cattenom 4 nuclear reactor, was down 80% at 2115 GMT, company data showed, but there was no early indication of major disruption to output nationwide.

The EDF strike, called by hard-left union CGT, started at 1900 GMT.

It is not expected to be as disruptive as a Sept. 19 walkout organised by four unions protesting plans to restructure EDF, which cut power generation by over 10%.

A CGT spokesman said earlier on Monday that the 24-hour stoppage was part of nationwide action by unions in several sectors against the government's pension reform, which are expected to hit schools, public transport, hospitals and courts on Tuesday.

President Emmanuel Macron's government plans to merge France's 42 pension systems into a single points-based system. Several professions fear they will lose out and have called for a nationwide strike following a Sept. 16 protest.

(Reporting by Paris Newsroom; Editing by Richard Lough)