Organisers say they expect up to one million people to demonstrate in 160 cities across the country.
The government says the new law will help bring down high youth unemployment levels, but opponents say it will entrench job insecurity.
Protests against the law earlier in the week resulted in hundreds of arrests.
University sit-ins
In Toulouse, in southwest France, up to 33,000 people took to the streets, while between 10,000 and 25,000 people demonstrated in Lyon.
Students, trade unions and left-wing political figures will attend a major rally in the capital, Paris, which is scheduled to start from Place Denfert-Rochereau at around 1330 GMT.
Marches are also expected to be held in dozens of other cities including Dijon, Marseille, Strasbourg and Bordeaux.
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