Paris ? Demonstrators hurled chunks of concrete at police, who responded by filling a picturesque Paris square with tear gas Thursday as protest marches over a contested jobs law erupted into violence.

Prime Minister Dominique de Villepin and union leaders, meanwhile, agreed to meet for the first time in the standoff.

The five main unions accepted an invitation from Villepin to meet today but said they would only restate their position and would not negotiate unless he withdraws the law. Unions plan a national strike on Tuesday.

Villepin agreed to meet with the unions despite their uncompromising stance, signaling a possible softening in his rigid stance as the government faces escalating protests that have wreaked havoc at universities, many shut down by strikes

Violence erupted in several cities Thursday as at least 220,000 students and other youths marched nationwide. France’s largest union of high school students, FIDL, estimated that 450,000 participated.