It is the sixth demonstration against the plan which triggered public outrage when it was announced last year.
Walkouts are jolting the entire country with serious disruptions to public transport as well as oil refineries.
Demonstrations have ended in some cities such as Nice, with the prefect reporting 6,000 protesters. Unions said 30,000 demonstrators showed up, according to the Le Figaro newspaper.
Protests in Lyon, in central France, were agitated by a masked group, forcing security forces to use tear gas to disband the group, said Le Figaro.
The capital will be the main site for demonstrations. Protesters gathered and started to march in Paris late in the day.
French intelligence is expecting between 1.1 million and 1.4 million protesters in 320 demonstrations, according to broadcaster BFMTV.
Paris alone will host up to 90,000 protesters, it said.
The reform plans include raising the retirement age from 62 to 64 in 2030 and requiring at least 43 years of work to be eligible for full pensions.
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