The French prime minister has indicated that he will not back down over his controversial youth labour law, despite union threats of a general strike.
Demonstrators hurled stones and bottles at police
In an interview for a French youth magazine, Dominique de Villepin said the law should be given a chance to work, but added it could be improved.
Unions have issued an ultimatum and are holding a meeting to plan a response.
More than 160 people were arrested on Saturday after clashes following a largely peaceful day of protests.
Work stoppages threat
The trades unions say the law will allow employers to exploit young people and have promised to "harden" their responses if the government does not make concessions.
Ahead of Monday's meeting, Bernard Thibault, head of the powerful General Labour Confederation (CGT), said: "If nothing moves, we will propose preparing a day of general work stoppages in the coming days. Conditions are such that it should be a success."
The government says the law will cut youth unemployment by making the labour market more flexible.
Unions said 1.5 million demonstrators took part in more than 150 rallies across the country against the government's First Employment Contract (CPE) on Saturday.
The interior ministry put the overall turnout at just over 500,000.
Speaking on Monday, President Jacques Chirac said that during the coming days it would be important to open a "constructive and confident dialogue" on the issue.
"I am confident that the sense of responsibility of employers and trade unions and the representatives of young people will lead them to go down this road, which is that of effectiveness and good sense," he told French TV.
Student and union leaders have been calling upon Mr Chirac to not sign the law, as he is required to do for it to take effect as expected in April.
Protesters are bitterly opposed to the new law, which allows employers to end job contracts for under-26s at any time during a two-year trial period without having to offer an explanation or give prior warning.
The government says it will encourage employers to hire young people, but students fear it will erode job stability in a country where more than 20% of 18-to-25-year-olds are unemployed.
The demonstrations came after a series of mass protests by students in dozens of French universities, which have severely disrupted classes.
Twenty-four people, including seven police officers, were injured in Saturday's violence, which lasted about six hours.
Clashes also erupted in other cities, including the port of Marseille, where demonstrators tried to set fire to the entrance to the town hall.