“We are profoundly shocked to learn about the death of blogger Sattar Behechti while in detention. We are calling on the Iranian authorities to explain the circumstances of his death,” said Vincent Floréani, deputy spokesperson at the Quai d’Orsay.
“France expresses its highest concern for the continuing degradation of human rights in Iran,” added Floréani.
Behechti, 35, was imprisoned on 28 October after being taken from his home in the city of Robat-Karim, just southwest of the capital Tehran.
In the last blog entry before he was arrested, Behechti said he had been harassed constantly by security service members over the telephone.
"Yesterday they threatened to tell my mother that she would soon be wearing black if I did not shut up," he wrote.
Behechti’s opposition activism took place on Facebook, which is blocked in Iran but can still be accessed through proxy websites. Iranian authorities have reportedly arrested a number of other Facebook activists.
After being pulled from his home, Behechti was brought to Kahrizak detention centre, where Iran held opposition activists following the country’s controversial presidential elections in 2009.
Kahrizak has become infamous for tales of torture and death. Behechti’s death cannot be independently verified, but his relatives received phone calls on Wednesday where they were told to pick up Behechti's dead body from the prison.
The People's Mujahedin of Iran, the principle opposition group outside of Iran, also announced the death of the blogger via a statement.
France has asked Iran to look into its detention centres countrywide, most notably Evin Prison. On 31 October, nine women began a hunger strike to protest against their treatment at the hands of prison guards.