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Masked young people clashed with police officers and set fires in cities across France on Tuesday as protests against a proposed hike in the retirement age took an increasingly radical turn.

President Nicolas Sarkozy pledged to crack down on "troublemakers" as clashes between youth and police broadened, saying he would ensure that "public order is guaranteed."

More than 200 protests and one-day strikes by workers in sectors across the French economy were planned around the country over a bill raising the retirement age from 60 to 62.

Initial estimates from the SNCF national railway operator and the Education Ministry suggested the number of public-sector strikers was diminishing after a week of disruptions by refinery workers, students and train staff that have snarled traffic, left drivers without gasoline and cancelled hundreds of flights.

In many cities, protesters were being joined by young people who appeared to be seizing an opportunity to lash out at police.

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.

A group of young people also knocked an Associated Press photographer off his motorbike and kicked and punched him as they rampaged down a street adjacent to the school.

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Youth clash with police forces in Lyon, central France, on Tuesday. ((Laurent Cipriani/Associated Press))

Nanterre has often seen student protests in past years and the latest clashes were reminiscent of 2005 riots that spread through poor housing projects nationwide with large, disenfranchised immigrant populations.

At the Place de la Republique in eastern Paris, young people pelted riot police with projectiles and torched garbage cans. Skirmishes between youths and police broke out in other cities.

It was the sixth national day of demonstrations over the planned pension reform since early September. Union leaders have vowed to keep up the pressure until the government scraps the unpopular plan, saying retirement at 60 is a fundamental social right that past generations fought hard to achieve.

Sarkozy called the reform his "duty" as a 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.

Fuel shortages

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 Tuesday out of Orly airport in Paris would be scrapped, and 30 per cent of flights out of other French airports, including the country's largest, Charles de Gaulle, serving Paris, would be cancelled.

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.

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French truck drivers staged go-slow operations on highways, rail strikes intensified and petrol stations ran out of fuel on Monday as protests escalated ahead of a Senate vote on an unpopular pension overhaul. ((Gonzalo Fuentes/Reuters))

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 on Tuesday 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 Tuesday to dislodge protesters blocking a fuel depot, which had been sealed off since Monday morning, local officials there said. No one was hurt in the operation, the officials said.

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 has been pushed back until later in the week so lawmakers have the time to examine hundreds of amendments brought by opposition Socialists and others.