Youths masked with scarves clashed with riot police in various areas, though no serious injuries were reported as of late Tuesday. Store windows were shattered, cars and garbage cans set on fire, and youths threw rocks at officers, notably in the center of Lyon, and in Nanterre, west of Paris. Students blocked the Place de la Republique in Paris and threw objects at police who forcefully opened the major intersection.

In Le Mans, in the Loire region, a middle school was burned to the ground some time after midnight Monday, authorities said. Though the incident took place long after students protested that day, many believe the school is an indirect casualty of the strikes.

In a show of support for the strikers, students have been piling up garbage cans and other large objects in front of their school entrances to prevent people from entering.

Government authorities, also dealing with flight cancellations and delays related to strikes by airport and transport workers, believed much of the violence came from individuals who were not part of the strike against President Nicolas Sarkozy's bill to revise the pension plan. The measure would raise the minimum retirement age from 60 to 62.

"As soon as there are youth [involved in the protests], we see a certain number of rioters infiltrate," said Justice Minister Michele Alliot-Marie on Europe 1 radio Tuesday.

Alongside the protest march in Paris, a group of boys and young men ages 17 to 19, some wearing hoods, scarves and dark sunglasses, said they often fought with riot police.

"Them and us — we don't get along," said Amine, 17, who did not provide his last name, because he said the police were looking for him. "There's this game of provocation between us and them. Both sides provoke each other."

Increasing reports of youth violence could turn public opinion against the protest movement, which has until now remained popular among the majority of French, according to polls. At least one major union called for a "pause" in the strike, in response to the violence, while strike organizers and politicians supportive of the movement asked for calm.

Union representatives said about 3.5 million people participated in the protests Tuesday. Government officials estimated about 1.1 million protesters marched throughout the country.

Government ministers and the president continue to call the pension bill the only solution to eliminating the retirement system's nearly $40-billion deficit by 2018. The Senate is expected to pass the retirement overhaul bill this week, but protesters plan to continue striking.

David Bouanida, 26, watched as Paris protesters marched by the insurance company where he works. He stood guard to make sure his parked car was not vandalized.

"I don't agree with the people on the street," Bouanida said. "They break, burn cars. We can't work, and clients can't come. And we're paid by commission."

He was waiting anxiously for senators to vote the pension bill into law. "Then it should be all over," he said.

*Lauter is a special correspondent.