Workers are angry about planned pension reforms that would see them retiring later or facing reduced payouts.
Teachers and transport workers have been joined by police, lawyers, hospital and airport staff, and other professions for a general walkout.
President Emmanuel Macron wants to introduce a universal points-based pension system.
That would replace France's current system, which has 42 different pension schemes across its private and public sectors, with variations in retirement age and benefits.
"What we've got to do is shut the economy down," said union official Christian Grolier of the Force Ouvrière (Workers' Force). "People are spoiling for a fight."
Since coming to power, Mr Macron has pushed through other reforms including relaxing labour laws and cutting taxes for businesses.
What's the latest?
By 16:00 local time (15:00 GMT), 280,000 people had joined demonstrations in 40 cities across France, not including Paris and Lyon.
The CGT union said workers had blocked seven of the country's eight oil refineries, potentially causing fuel shortages if the strike continues.
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