French oil workers defied the government's demand Monday to get back to work and end fuel shortages, stepping up their fight against President Nicolas Sarkozy's retirement reforms, as youths faced off against riot police and truckers joined the protests.

Airlines, meanwhile, were told to drastically cut back their flights into France on Tuesday, when the next national street protests are planned, and severe disruptions to air travel, public transport, schools and other facilities are expected.

Strikers have blockaded a dozen French refineries and numerous oil depots in the last week as part of the widespread protests over Sarkozy's plan to raise the retirement age to 62, which the French Senate will debate on Wednesday.

Fearful motorists have flocked to gas stations in panic and found many empty, while aviation authorities told short-haul planes coming in from other European destinations to bring enough fuel to get back.

Striking workers piled up tires and set them ablaze Monday in front of a refinery at Grandpuits, east of Paris, after authorities issued a legal order insisting that some strikers reopen the facility. Workers said they would refuse, as curls of heavy black smoke wafted into the air.

Other employees and residents formed a "human chain" to prevent the refinery workers from entering the plant, and union leaders said they expected police to intervene.

Dozens of oil tankers remained stuck in the Mediterranean, anchored outside Marseille's two oil ports, where workers have been on strike for more than three weeks to protest a planned port reform as well as the retirement changes.

Prime Minister Francois Fillon pledged Sunday to do what's necessary to prevent fuel shortages, saying the government won't allow such shortages to hurt the French economy. The head of France's petroleum industry body said fuel reserves were "enough to keep us going for a few weeks."

Jean-Louis Schilansky, president of the Petrol Industries Association, warned however that if the strikers continue to block fuel depots and if the nation's truckers join the movement, "then we will have a very big problem."

Truckers did join the fray, staging organized slowdowns Monday aimed at snarling highway traffic. French TV showed images of cars and trucks on a "Snail Operation" - driving at a snail's pace on the main highway between Paris and the northern city of Lille, with red union flags waving out the windows.

The government opened a crisis coordination center Monday in the Interior Ministry, focusing on the conflict in the oil sector.

Countries across Europe are cutting spending and raising taxes to bring down record deficits and debts from the worst recession in 70 years. Labor leaders, students and civil servants are fighting back.

French youth burned tires and cars, set up blockades and clashed with riot police Monday outside some high schools in Paris and nearby suburbs.

Students from Lycee Joliot Curie in the Paris suburb of Nanterre tried to blockade their school, with about 100 facing off against police, who responded with rubber bullets. In all, 261 schools were blocked Monday, the education ministry reported.

Rail unions, which have been on strike since early last week, have prolonged their walkouts through Tuesday to coincide with more than 200 street protests - the sixth nationwide protests in about a month.

Union leaders also called for support strikes from other sectors, including energy, postal workers and private commerce, as well as from employees at Eurotunnel, which runs freight and passengers under the English Channel to London.

France's DGAC civil aviation authority ordered airlines to cancel 50 percent of their flights Tuesday out of Paris' Orly airport, and 30 percent at other airports nationwide, including Paris' largest airport, Charles de Gaulle. Most disruptions were expected to hit short- and medium-haul flights.

Airport operator ADP said strikes by oil workers at Charles de Gaulle had already begun causing some delays on Monday. Spokesman Jerome Landras said travelers should contact their airline to check on flight details.

Train traffic also continued to suffer from the nearly weeklong strike. France's SNCF railway operator said about half its high-speed TGV trains had been canceled Monday, but traffic on the Eurostar between Paris and London was normal.