Workers are angry at being forced to retire later or face reduced pensions.
School and transport workers have been joined by police, lawyers, and hospital and airport staff for a general walkout that could include millions of people.
France’s largest nationwide strike in years was agreed by unions unhappy with President Emmanuel Macron’s plans for a universal points-based pension system.
“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.”
Interior Minister Christophe Castaner said on the eve of the strike he expected almost 250 demonstrations nationwide, some of which he said could turn violent.
“We know there will be lots of people in these protests and we know the risks. I have requested that systematically when there is rioting or violence we make arrests immediately,” he said.
On Paris’s famous Champs-Élysées boulevard, riot police began searching pedestrians’ bags before dawn, and shops on a planned protest route have been ordered to close.