"All the unions are calling to make 28 March a day of demonstrations, strikes and work stoppages," Rene Valadon, confederal secretary of the Force Ouvriere union, said on Monday after a meeting of France's main unions, student and high school groups.
Another union leader, Gerard Aschieri of the Federation of United Unions, said public and private workers would go "hand in hand" during the day of action.
Despite nationwide protest marches over the weekend, Dominique de Villepin, the prime minister, has stood firm on the First Job Contract (CPE) law but also called for dialogue to improve it.
He held meetings with students and employers on Monday to discuss their concerns and promote the law.
Jacques Chirac, the French president, on Monday backed his prime minister over the law, which allows employers to fire people under 26 for any reason during a two-year trial period.
Chirac said the CPE showed the government's willingness to fight youth unemployment, which is 23% in France, more than twice the national rate.
The unrest may have damaged
Villepin's presidential prospects
"The challenge ... is to open a constructive and confident dialogue in this spirit which can allow improving the CPE," Chirac told a news conference with Jordan's King Abdullah.
"I know this is the prime minister's and the government's willingness and I can only approve of it," Chirac said.
Opposition to the law has provoked a crisis for Villepin, which could harm his chances of running for president in 2007 and damage the UMP party.
Opinion polls show his popularity has slumped in recent weeks, and a poll for the BVA organisation on Monday showed that 60% of French voters want the law withdrawn.
Student groups and union leaders say the CPE would create a generation of disposable workers without job security.
In an announcement likely to inflame supporters of the CPE protests, the Sud-PTT union said one of its members was critically ill and in a coma after being injured in clashes with police that followed Saturday's march through Paris.
The 39-year-old man was hospitalised after being injured in the face during clashes between police and protesters at the Place de la Nation in eastern Paris, police said in a statement. He was in serious condition, it added.
The Paris prosecutor's office has opened a preliminary inquiry, the statement said.
Students have clashed with the
police in Paris and elsewhere
Bernard Allaire of Sud-PTT denounced police violence and told Reuters: "His situation is worse than alarming. No one is allowed to see him except his immediate family."
Police clashed with protesters in a suburb outside Paris on Monday and more student rallies are planned for Tuesday.
The French work code contains rigorous standards for firing employees.
But Villepin hopes to use the measure to lower the 23% unemployment rate among the nation's youths, a figure that rises to about 50% in depressed suburban neighbourhoods where unrest erupted last year, fueled by discrimination and joblessness.