* Supply cut at fuel pipeline to Paris
* Airport worker strike grounds some flights
* Police arrest dozens in student protests
(Adds arrests and strike call by Air France union)
By Catherine Bremer
PARIS, Oct 15 (Reuters) - Striking French oil refinery workers shut down a fuel pipeline supplying Paris and its airports on Friday and airport workers grounded some flights as protests mounted to derail an unpopular pension reform.
France's airport operator played down worries of fuel shortages, but strikes at all of the country's 12 refineries and fuel depot blockades prompted motorists to stock up on petrol.
Truck drivers also were set to join the fray as momentum built for a day of street rallies on Saturday.
The widening protests have become the biggest challenge facing President Nicolas Sarkozy, who is struggling with low popularity ratings as he tries to appease financial markets by stemming a ballooning pension shortfall.
"This movement is deeply anchored in the country," CGT union leader Bernard Thibault told LCI television. "The government is betting on this movement deteriorating, even breaking down. I think we have the means to disappoint them."
Police broke up blockades at fuel depots in the south but Air France was forced to cancel some flights from Paris as runway workers at Orly airport halted work.
A nationwide strike that could hit various industries is planned for Tuesday, a day before the Senate is due to vote on a bill to make people work longer for their pensions.
One of the main unions representing Air France workers called for three days of strikes from Monday.
The main trucking union called on drivers to join Tuesday's strike, although they may not be able to use their companies' vehicles to block roads.
"We're filling up at petrol stations to save the fuel we have in depots as much as possible," said one trucker in Paris's southern suburb of Rungis. "But if this carries on, we won't be able to last too long either. We'll have to go fishing."
Turnout among striking rail workers dropped to 15 percent on Friday, from 40 percent earlier in the week, but union leaders hope to galvanise the public for next week's action with the same force that saw a 1995 pension bill crushed by 24 days of protests. Next Tuesday's strike could hit various sectors.
The company operating a key fuel pipeline from Le Havre in northern France to Paris said a refinery strike had cut supply to the capital. A spokeswoman for Paris airport operator ADP said they had several days of fuel stocks.
At Donges in western France, striking refinery workers built a barricade of burning tyres as they waited to confront police.
The best chance Sarkozy's opponents have of bringing down his pension bill is if strikes at oil refineries continue and start to threaten fuel supply, or if youths hit the streets en masse and set off violent scuffles.
TV footage showed riot police using teargas to contain young protesters in the southern city of Lyon.
About 260 people were arrested throughout the country, police said, describing them mostly as "troublemakers", while about a dozen officers were injured.
In Paris, police got orders to stop using "flashball" riot-control pellets, a type of rubber bullet, to quieten crowds after a secondary school student was badly injured on Thursday.
Students at hundreds of schools joined the protest movement in force from Thursday, shouting anti-Sarkozy slogans. Dozens have been arrested and on Friday more were barred by riot police from nearing the prime minister's offices.
"What will they become if tomorrow there is no money to pay their pensions? I ask each of them to ask themselves the question," Sarkozy said.
Jean-Claude Gaudin, mayor of the southern port of Marseille, where several hundred youths rallied on Friday and unrelated strikes by port workers and garbage collectors have clogged up city life all week, said teenagers should go back to school.
"Those who are urging secondary school students into the street are irresponsible. It's staggering," he said.
Polls show two-thirds of people oppose Sarkozy's plan to raise the minimum retirement age to 62 from 60 and change the age at which people can retire on a full pension to 67 from 65.
The government has been at loggerheads with unions for months over the issue and five rounds of strike action since the summer have disrupted public transport and air travel.
The strikes have had a negligible effect on the economy but have aroused concern among financial analysts about whether France will struggle to push through broader austerity measures necessary to bring down its deficit. (Additional reporting by John Irish, Brian Love, Helen Massy-Beresford, Thierry Leveque and Mathilde Cru; editing by Andrew Dobbie)