French unions threatened a one-day general strike and socialists warned Sunday that protests would increase unless the government rescinds a law weakening job protections for young people. Prime Minister Dominique de Villepin said the measure will stand.

Protesters who have been demonstrating for weeks have urged President Jacques Chirac against signing the measure into law. Sixteen universities are on strike in protest over the labor measure and dozens of others have been disrupted. The law is to take effect in April if Chirac signs.

The law is meant to encourage employers to hire because they can more easily fire. It allows for dismissal within the first two years of employment without giving a reason. The French work code contains rigorous standards for firing employees.

In the largest series of protests yet, 500,000 people rallied on Saturday in some 160 French cities. At the Paris rally, police loosed water cannons and tear gas on rioting students and activists rampaged through a McDonald’s and attacked storefronts.