France braced Monday for widespread disruptions to train, plane and subway traffic in a strike over the government's new labor law as Prime Minister Dominique de Villepin and opponents of the measure entered a crucial week in their ongoing standoff.

Unions for national trains operator SNCF urged rank-and-file to walk off the job starting Monday evening. The strike was expected to swell to full force Tuesday as other unions join in.

The week was shaping up as a critical test for Villepin, who has largely refused to budge on the labor law that he insists is needed to bring down sky-high youth unemployment rates. Many opponents fear it will damage coveted job security in France.

Labor and student groups vowed to press ahead with the one-day strike and more protests unless the government cancels the law that has sparked violent clashes with police and shut down universities.

"CPE: The fateful week," read the banner headline on Monday's edition of Le Figaro newspaper, using the French acronym for Villepin's contested first job contract.

The law lets companies dismiss workers under 26 without cause during their first two years on the job — a provision the government hopes will make it easier for employers to hire younger workers.

Union leaders said they would meet Wednesday to decide on the next step, and left open a threat to extend the strike.

France's constitutional council, responding to an appeal from the opposition Socialists, was to rule Thursday on the legality of the new law, which passed parliament early this month but still needs President Jacques Chirac's signature to take effect, as planned, by April.

France's top union of students, UNEF, called for dialogue with Villepin — but continued to insist that the government shelve the law first as a precondition to talks.

The National Student Coordination, another grouping of university students, has demanded the conservative government's resignation. At a meeting in southeastern Aix-en-Provence, the group vowed more protests after the strike by blocking roads and train tracks on Thursday.

About 200 demonstrations are planned across the country Tuesday, with the largest winding through Paris. The protest will be the sixth in about two weeks in the capital. Most have turned violent, with youths clashing with police and, last week, even attacking student demonstrators.

Paris police, in a statement, said regional and commuter rail networks will be under "increased surveillance" on Tuesday and that "preventive identity checks of individuals susceptible to participate in violence" would be authorized.

Most Air France unions are taking part in the strike. The civil aviation authority said some flights were likely to be canceled, though the exact scope of disruption is unclear. The authority urged travelers to contact carriers directly for information.

In Paris, half of subway trains were expected to run, while the suburban RER commuter lines were to face greater disruptions, the RATP transport authority said. SNCF said two of three trains were expected on main routes, but Eurostar service to London and Thalys trains to Brussels would not be affected.

Though some universities and high schools have already been disrupted or completely shut down, more schools were to close their doors Tuesday during a teachers strike.