Violence has flared on protest marches in opposition of President Nicolas Sarkozy's pension reform plans.
France's CGT union says 3.5 million people have joined nationwide street demonstrations, although government estimates are much lower.
Refinery workers, airport staff, train drivers, teachers, postal workers and guards who supply cash machines walked out.
Students set off rowdy protests. Cars and litter bins were torched in the southern city of Lyon, while cafe chairs were used to smash shops and bank windows.
Police used tear gas in the Paris suburb of Nanterre to break up demonstrators.
President Sarkozy has been in the seaside town of Deauville for talks with the leaders of Russia and Germany. He appealed to protesters to show restraint.
The head of the CFDT union, Francois Chereque, also called for non-violent protests. "I am making a clear and simple appeal... Let's keep this peaceful," he said at the head of a march in central Paris.
This week will be critical for Sarkozy's reform, which the government says is vital to curb a gaping pension deficit and safeguard a coveted AAA credit rating which allows France to finance its large public debt at the lowest market rates.
A majority of voters resent the plan to raise the minimum and full retirement ages by two years to 62 and 67 respectively, and unions are demanding negotiations on the pensions overhaul.
Most analysts expect the legislation will pass within days and the protests will fizzle out. But the unions, which defeated pension and labour reforms with strikes in 1995 and 2006, say they will press on regardless.
The protests in France are the biggest and most persistent challenge to economic reforms anywhere in Europe, as governments struggle to curb budget deficits and reduce debt mountains.