Paris — Masked youths clashed with police and set fires in cities across France today as protests against a proposed hike in the retirement age took an increasingly radical turn. Hundreds of flights were canceled, long lines formed at gas stations and train service in many regions was cut in half.

President Nicolas Sarkozy pledged to crack down on "troublemakers" and guarantee public order, raising the possibility of more confrontations with young rioters after a week of disruptive but largely nonviolent demonstrations.

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Sarkozy also vowed to ensure that fuel was available to everyone. More than 1,000 gas stations are currently shuttered nationwide.

The protesters are trying to prevent the French parliament from approving a bill that would raise the retirement age from 60 to 62 to help prevent the pension system from going bankrupt. Many workers feel the change would be a first step in eroding France's social benefits — which include long vacations, contracts that make it hard for employers to lay off workers and a state-subsidized health care system — in favor of "American-style capitalism."

Sarkozy's conservative government points out that 62 is among the lowest retirement ages in the world, the French are living much longer and the pension system is losing money. The workers say the government could find pension savings elsewhere, such as by raising contributions from employers.

The Paris airport authority warned on its website and in signs at the airports: "Strike on Oct. 19. Serious difficulties expected in access to airports and air traffic." France's DGAC civil aviation authority said up to half of flights today out of Paris' Orly airport would be scrapped, and 30 percent of flights out of other French airports, including the country's largest, Charles de Gaulle, serving Paris, would be canceled.

Most cancellations were on short- and medium-haul domestic and inter-European flights. The walkout by air traffic controllers was expected to last one day, with flights expected to return to normal on Wednesday.

In Metro Detroit, Metro Airport is reporting no problems so far with its one flight daily to France. Airport spokesman Mike Conway said the Delta/Air France flight was on schedule Monday and is set to fly again this evening. Delta, however, has warned customers the strike could affect flights. Conway said the airline is making sure its planes have enough fuel to land at alternate sites if required.

At the airport in the Atlantic city of Bordeaux, scores of protesters blocked the entrance for several hours this morning In Paris, huge crowds started marching from the Place d'Italie in the south toward the gilded-domed Invalides, where Napoleon is buried. The protest appeared peaceful, but law-enforcement officials were bracing for possible confrontations with youth. Police estimated the crowd at 60,000, down from 65,000 at a similar march last week.

At a high school in the Paris suburb of Nanterre, closed because of earlier violence, a few hundred youths started throwing stones from a bridge at nearly as many police, who responded with tear gas and barricaded the area. It was not immediately clear if there were injuries or arrests. Youths also knocked an Associated Press photographer off his motorbike and kicked and punched him as they rampaged down a street adjacent to the school. Another AP photographer was hit in the face by an empty glass bottle in Lyon, where protests turned violent and rioters smashed several store windows.

The violence recalled student protests in 2006 that forced the government to abandon a law making it easier for employers to hire and fire young people. Those protests started peacefully but degenerated into violence, with troublemakers smashing store windows and setting cars and garbage cans ablaze.

The specter of 2005 riots that spread through poor housing projects nationwide with large, disenfranchised immigrant populations was also present.

At the Place de la Republique in eastern Paris today, young people pelted riot police with projectiles, while youth in the central city of Lyon torched garbage cans and cars as police riposted with clouds of tear gas.

It was the sixth national day of demonstrations over the planned pension reform since early September. Union leaders have vowed to keep up pressure until the government scraps the unpopular plan and opens negotiations.

Sarkozy called the reform his "duty" as head of state and said it must go through to save France's generous but money-losing pension system. The protests in France come as countries across Europe are cutting spending and raising taxes to bring down record deficits and debts from the worst recession in 70 years.

Strikes by oil refinery workers have sparked fuel shortages that forced at least 1,000 gas stations to be shuttered. Other stations saw large crowds. At an Esso station on the southeast edge of Paris this morning, the line snaked along a city block and some drivers stood with canisters to stock gasoline in case of shortages.

Sarkozy said such shortages "cannot exist in a democracy."

"There are people who want to work, the immense majority, and they cannot be deprived of gasoline," he insisted.

Police in the northwestern town of Grand-Quevilly intervened early this morning to dislodge protesters blocking a fuel depot, which had been completely sealed off since Monday morning, local officials there said. No one was hurt in the operation, the officials said.

Truckers have joined the protest, running so-called "escargot" operations in which they drive at a snail's pace on highways. Today, about 20 truckers blocked an oil depot in Nanterre west of Paris operated by oil giant Total, turning away fellow truckers coming to fill up with gasoline. Police stood by but did not intervene.

Students entered the fray last week, blockading high schools around the country and staging protests that have occasionally degenerated into clashes with police.

Across the country, 379 high schools were blocked or disrupted today to varying degrees, the Education Ministry said. It was the highest figure so far in the student movement against the retirement reform. Student movements have forced previous governments to back off planned reforms in the past, and student leaders hope these protests will prove as successful.

The head of the UNEF student union, Jean-Baptiste Prevost, said the students "have no other solution but to continue."

"Every time the government is firm, there are more people in the street," he told i-tele news channel, predicting a large turnout for today's street marches.

With disruptions on the national railway entering their eighth consecutive day today, many commuters' patience was beginning to wear thin. Only about one in two trains were running on some of the Paris Metro lines, and commuters had to elbow their way onto packed trains.

In a statement posted on its website, the SNCF railway operator said only about half the regularly scheduled high-speed TGV trains linking Paris to regional French cities was operating today, while fast trains between regions was slashed by 75 percent. The Eurostar, which links Paris to London via the British Channel tunnel, is unaffected, the statement said.

In the Mediterranean port city of Marseille, strikes by garbage collectors have left heaps of trash piled along city sidewalks. But still, the piles of rotting garbage don't appear to have diminished labor union support in a city that has long had an activist reputation.

"Transport, the rubbish, the nurses, the teachers, the workers, the white collar, everyone who works, we should all be united. If there is no transport today, we're not all going to die from it," said 55-year-old resident Francoise Michelle.

Sarkozy has stressed that 62 is among the lowest retirement ages in Europe, the French are living much longer and the pension system is losing money.

The measure is expected to pass a vote in the Senate this week. Slated to take place on Wednesday, it's been pushed back until later in the week so lawmakers have the time to examine hundreds of amendments brought by opposition Socialists and others.

Student leaders have called for a demonstration in front of the Senate on Wednesday and another round of strikes at high schools and universities on Thursday.