State-run rail said only two out of three of its high-speed trains, only 40 percent of other long-distance trains and about half its suburban lines are operating.
Airports are also expected to be hit by stoppages, and passengers have been urged to check with their airlines.
Some postal workers, teachers and media employees said they would join the strike, with hundreds of thousands expected to attend protest rallies.
Previous marches have degenerated into running battles between police and rioters.
On Thursday gangs of youths from Paris's high-immigration suburbs smashed windows, set fire to cars and mugged demonstrating students.
The strike comes on the fifth day of nationwide demonstrations against Prime Minister Dominique de Villepin's First Employment Contract initiative.
Mr De Villepin is potentially to stand in next year's presidential election, and has defended the plan, saying he hopes it will reduce youth unemployment from the current 23 percent.
He said he hopes the plan will encourage employers to hire younger workers by giving them more flexibility.
But union and student leaders say it will create a generation of "throwaway workers" because it makes it easier to dismiss young workers in a trial two-year period.
They say it is a breach of hard-won labour rights and will make it more difficult than ever for young people to find long-term jobs.
Transport workers were the first to strike, stopping work on some railway and subway lines on Monday night.
In an increasingly bitter three-week struggle, the government has refused to cede to opposition demands to scrap the CPE, a contract for under 26-year-olds that can be terminated without explanation during a two-year trial period.
Opinion polls show almost two-thirds of French people oppose the CPE and unions said 135 rallies are planned across France.
Police said they would be out in force because of fears violence could flare, as it did during protests last week.
Mr Villepin, 52, a former foreign minister who has never been elected to public office, has invited unions and student bodies to meet him for talks, however it is not clear whether they will accept the invitation.
Opposition Socialist Party leader Francois Hollande said the invitation would be pointless if Mr Villepin is not ready to scrap the youth job contract.