VILLIERS-LE-BEL, France — Youths rampaged for a third night in the tough suburbs north of Paris and violence spread to a southern city late Tuesday as police struggled to contain rioters who have burned cars and buildings and – in an ominous turn – shot at officers.

A senior police union official warned that “urban guerrillas” had joined the unrest, saying the violence was worse than during three weeks of rioting that raged around French cities in 2005, when firearms were rarely used.

Bands of young people set more cars on fire in and around Villiers-le-Bel, the Paris suburb where the latest trouble first erupted, and 18 people were detained, the regional government said. In the south, 10 cars and a library went up in flames in Toulouse, police said.

Despite the renewed violence, France’s prime minister said the situation was calmer than the two previous nights.

The government was striving to keep violence from spreading in what was shaping up as a stern test for new President Nicolas Sarkozy. The unrest showed anger still smolders in France’s poor neighborhoods, where many Arabs, blacks and other minorities live largely isolated from the rest of society.

The trigger was the deaths Sunday of two minority teens when their motor scooter collided with a police car in Villiers-le-Bel, a blue-collar town on Paris’ northern edge.

Residents claimed the officers left without helping the teens. Prosecutor Marie-Therese de Givry denied that, saying police stayed on the scene until firefighters arrived.

Rioting and arson quickly erupted after the crash. The violence worsened Monday night as it spread from Villiers-le-Bel to other impoverished suburbs north of the French capital. Rioters burned a library, a nursery school and a car dealership and tried to set some buildings on fire by crashing burning cars into them.

Police reinforcements were moved into trouble spots north of Paris on Tuesday. Helicopters flew overhead, shining powerful spotlights into apartment buildings to keep people from leaving their homes.

“The situation is under control,” said Denis Joubert, director of public safety for the region surrounding Villiers-le-Bel.

Prime Minister Francois Fillon, who was briefed by police in Villiers-le-Bel, said things were “much calmer than the previous two nights, but we feel that things are still fragile, and we need a large preventative force on the ground so that what happened last night does not happen again.”

Patrice Ribeiro of the Synergie police union said rioters this time included “genuine urban guerrillas,” saying the use of firearms – hunting shotguns so far – had added a dangerous dimension.

Associated Press writers Angela Charlton in Villiers-le-Bel and John Leicester contributed to this report