France today demanded the immediate release of a 23-year-old student from Lille who has been jailed in Iran on spying charges after she took photographs at a demonstration last month.

President Sarkozy dismissed as "pure fantasy" the charges against Clotilde Reiss, a politics student and an assistant teacher at Isfahan University, who was detained at Tehran airport last week on her way home to France.

"Let me say in the clearest and simplest way possible: we demand the release of our compatriot," Mr Sarkozy told a joint news conference with Luiz Inácio Lula da Silva, the President of Brazil.

"These accusations of espionage are pure fantasy and there is no reason for them. I do not doubt for an instant that she will be released very, very soon."

Mr Sarkozy added that he disapproved of such "methods".

It emerged last night that Ms Reiss, a graduate student at the eminent Lille Political Sciences Institute, had been arrested last Wednesday as she was leaving for home via Beirut after a five-month stint at the university. The French authorities made her arrest public last night after failing to win her release.

The Foreign Ministry summoned the Iranian Ambassador to Paris yesterday to demand her released.

France also alerted its European Union partners and said it was seeking an EU-wide response to Iran after the arrest of the academic and the detention of staff at the British Embassy in Tehran. All but one of the nine embassy staff detained in Tehran have now been freed.

Iran alleges that Ms Reiss took a photograph with her mobile telephone of demonstrations in Isfahan against the re-election of President Ahmadinejad and e-mailed it to a friend in Tehran. "That is not espionage and cannot be so. The accusation is absurd," Dr Kouchner said. "She was a classic young student who was doing her job, who witnessed protests like millions of Iranians."

Until now, France has not been attacked, as Britain has been, by the Iranian authorities. Yesterday, in the Alpine resort of Evian, President Sarkozy told Gordon Brown that Britain had France's total backing in Iran. "The Iranian people deserve better than the leaders they have today," he said.

Bernard Kouchner, the French Foreign Minister, said that France wanted Ms Reiss's case to be resolved rapidly and did not intend to "make a big issue between two countries out of this".

He added: "I do not think the Iranian authorities can think for a second that what is happening in their streets — the opposition to a government — can be settled by arresting innocent people. An innocent French person, what's more."

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