(Adds increased security, paragraph 2, Fillon, 11-13)

By Crispian Balmer

PARIS, Dec 16 (Reuters) - Police found explosives hidden in one of Paris's busiest department stores on Tuesday after receiving a tip-off from a previously unknown group demanding the withdrawal of French troops from Afghanistan.

Interior Minister Michele Alliot-Marie said five "relatively old" sticks of dynamite were discovered the Printemps store and announced she would bolster security in major French cities in the run-up to Christmas.

A group calling itself the Afghan Revolutionary Front warned in a letter to a French news agency that it had planted several bombs in the Printemps store on the Boulevard Haussmann, which is usually packed with shoppers at this time of year.

Police evacuated the building and found explosives concealed in a lavatory cistern. There were no detonators, suggesting the aim was to spread alarm rather than cause death.

In its letter, the group warned France it would "return to action in your big capitalist stores and this time without warning" unless French forces pulled out of Afghanistan by the end of February, 2009.

Alliot-Marie said French security forces had never heard of the group and cautioned about its possible motives.

"Anything can be written in a text," she told reporters. "There might be things that are designed to send one off in the wrong direction."

TROOPS IN AFGHANISTAN

Police said there was no mention of Islam in the letter, adding that recent attacks in Europe tied to Iraq and Afghanistan had been launched without specific prior warning.

France has more than 2,600 troops stationed in Afghanistan fighting Taliban forces. Last month, a militant group warned in a video aired on Al Arabiya television that it would attack Paris unless the soldiers went home.

French stores and metro stations were bombed several times in the 1980s and 1990s, with Algerian or Middle Eastern militants often claiming responsibility.

Prime Minister Francois Fillon said on Tuesday there was a high risk of more attacks against France.

"It is high because France is a country which is engaged in the defence of freedom across the world and because France has forces in several places where we are fighting terrorist organisations," he said.

"France will not give in to terrorist threats," he added.

The discovery of the explosives came at a particularly bad time for the French retail sector, which notches up much of its annual profits in pre-Christmas trading but faces a downturn this year because of the financial crisis.

"The goal is to frighten people," saidValerie Plagnol, chief strategist at CM-CIC Securities. "Security will be boosted in stores, and sales could be affected."

Printemps, formerly part of the PPR retail and luxury goods group, was sold to an investor consortium in 2006.

Italian tourist Ilaria de Pasqua shrugged off the incident after she was prevented from entering Printemps by police.

"There are lots of shops. I am going to go to Galeries Lafayette instead," she said. (Additional reporting by Gerard Bon, Anna Willard and James Mackenzie; editing by Andrew Roche)

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