In an interview with broadcaster France Info, employees' leader Philippe Martinez justified the strike, saying national-level mobilisations were needed to discuss the "bad" law, EFE news reported.
The striking unions have planned over 110 protests in cities across the country.
During the most recent protest on July 5, tens of thousands of demonstrators turned out, a number that paled in comparison to the hundreds of thousands who protested on March 31.
Although the police put the turnout at 390,000, trade unions claimed it was 1.2 million.
Strikes also affected air traffic beginning early Thursday. The French civil aviation authority DGAC had forewarned Tuesday that 15 per cent flights at the Orly and Charles de Gaulle airports in Paris, besides the one at Beauvais, 80 km from the capital, would have to be cancelled.