Powerful unions CGT and CFDT called for mass demonstrations against the government's plans to extend the number of years employees must work to get a full pension to 41 from 40, and to give companies more scope to by-pass the 35-hour week.
"Employers will be able to do almost anything they like unilaterally. The government is opening the way for total deregulation," said Francois Chereque, head of CFDT, in an interview on France Info radio.
Chereque predicted the demonstrations would be bigger than on the last major day of strikes, May 22, thanks to the participation of private as well as public sector workers.
Unions said 700,000 marched against pension reforms then, while police put the figure at 300,000.
Bernard Thibault, head of CGT, said he expected over a million people to take part in demonstrations in about 120 towns and cities across France on Tuesday.
Train traffic was disrupted early in the day on regional networks in the south while bus and tram services in several major cities including Nice were hit, according to radio reports. Long-distance trains were running normally.
The Paris metro was also operating normally but there were some disruptions on the regional RER network and commuter trains. The road traffic authority reported 166 km (103 miles) of traffic jams around Paris as commuters avoided the rail network.
Figures were expected later in the day on the proportion of employees taking part in the strike.
The unions were taking a gamble by calling for strikes and mass marches after months of protests by teachers over job cuts, port workers over reforms, fishermen and truckers over fuel prices and other professions over pension reforms.
The May 22 strikes did little to slow the government's pension reform drive despite the big demonstrations, and the unions will have to stave off protest fatigue on Tuesday to make an impact.
Welfare Minister Xavier Bertrand is due to present a new bill to the cabinet on Wednesday reforming the 35-hour week, introduced 10 years ago when the Socialists were in power and denounced by the centre-right Sarkozy as an economic disaster.
Bertrand's proposal would open the way for negotiations at company rather than industry level on maximum numbers of overtime hours and compensation for them.
The unions say this would give greater power to bosses, who tend to be stronger in negotiations within individual companies than at industry level where they face the full weight of the unions.
But unlike on May 22, when all eight major unions were united in their call to action, CGT and CFDT were isolated on Tuesday after falling out with other unions over an agreement they signed with bosses' group MEDEF.
The agreement was mostly aimed at improving labour relations, but it contained a clause on new ways to organise overtime which the other unions said allowed the government to challenge the 35-hour week.
Last Mod: 17 Haziran 2008, 17:00