French protesters blockaded Marseille's airport, truckers tied up highways and Lady Gaga canceled concerts in Paris ahead of a tense Senate vote Thursday on raising the retirement age.

A quarter of the nation's gas stations were out of fuel despite President Nicolas Sarkozy's orders to force open fuel depots barricaded by striking workers.

Gasoline shortages and violence on the margins of student protests have heightened the standoff between the government and labor unions who see retirement at 60 as a hard-earned right.

Students barricaded a Paris high school and planned protests nationwide later Thursday, as the Senate wraps up protracted debate on a reform that Sarkozy calls crucial to his presidency.

Student protests have forced the government to its knees in the past, and in recent days some have degenerated into violence. Rioters threw stones at police Wednesday night in the city of Lyon.

The French government _ like many heavily indebted governments around Europe _ says raising the retirement age and overhauling the money-losing pension system is vital to ensuring that future generations receive any pensions at all.

French unions say the working class is unfairly punished by the pension reform and that the government should find money for the pension system elsewhere. They fear this reform will herald the end of an entire network of welfare benefits that make France an enviable place to work and live.

"We cannot stop now," Jean-Claude Mailly, head of the Workers' Force union, said Thursday of the protest movement.

In Marseille, hundreds of workers blocked all access to the main airport for about three hours early Thursday. Passengers tugged suitcases along blocked roads as they hiked to the terminal, before police came in and the protesters dispersed.

Protester Leshmi Taguelmint of the CGT trade union, remained determined. "We will continue our action, for the time being we have the whole population behind us and we will continue," he told AP Television News.

Protesters in cars and trucks blocked several highways around the country Thursday, from near Calais in the north to the Pyrenees in the south, according to the national road traffic center.

Interior Minister Brice Hortefeux stood firm, lashing out at "certain people who take pieces of our territory for battlefields."

Speaking on Europe-1 radio Thursday morning, Hortefeux said 1,901 people have been detained since early last week.

Hortefeux insisted that the country has several weeks of gasoline reserves and that "the trend is toward improvement" in supplies. Still, the environment minister said there were more than 3,000 gas stations empty of fuel.

Families around the country are particularly on edge because of the gasoline shortages because school vacations start Friday.

Authorities, however, are hoping that the vacations cool off student tempers. On Wednesday, hooded youths smashed store windows in the Paris suburb of Nanterre and the city of Lyon, as riot police sprayed tear gas in response.

The Senate vote on the measure could come as soon as Thursday, or the debate could drag on for another day or two. Opposition Socialists proposed more than 800 amendments to the pension reform bill approved by the lower house of parliament last month, and the Senators must debate and vote on each one. As of Thursday morning, they still had more than 200 left.

Finance Minister Christine Lagarde said both the strikes and the violence were taking an economic toll.

"I'm calling on people to be responsible, in particular those who are having a roaring time blocking access and breaking things," she said Wednesday night on TF1 television. "It's serious for our country."

Lady Gaga's website says the singer postponed two Paris concerts "as there is no certainty the trucks can make it" to the show.

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AP Television News reporters Oleg Cetinic in Paris and Jonathan Shenfield in Lyon contributed to this report.