Women's rights activists say Esha Momeni lives in the United States and was in Iran for research on the women's movement in the Islamic Republic as part of her university studies when she was detained in the capital on October 15.
In the first comment by the judicial authorities on the case, judiciary spokesman Alireza Jamshidi said Momeni was being held in Tehran's Evin prison.
"The charge against her is crime against national security and her case is currently under preliminary investigation," Jamshidi told a news conference, referring to a common charge against dissenting voices in the Islamic Republic.
He gave no further details about Momeni or her case.
Iranian women's rights campaigners said Momeni was working on a film and had interviewed activists in Tehran as part of her studies in California. She came to Iran about two months ago.
"It is definitely unwarranted … she hasn't done anything wrong," activist Sussan Tahmasebi said about the accusation against Momeni. "Hopefully she will be released soon."
She said Momeni was born in the United States and held both Iranian and U.S. citizenship.
Activists say dozens of them have been detained since they launched a campaign in 2006 to collect 1 million signatures in support of demands to end what they see as legal discrimination of women in Iran.
Western diplomats see the detentions of women's rights activists as part of a wider clampdown on dissent by President Mahmoud Ahmadinejad's government as it faces Western pressure over its disputed nuclear program.
Most of the women detained were freed within a few days or weeks. Iran rejects charges of abuse.
The campaign's website www.forequality.info said Momeni was a graduate student of the School of Communications, Media and Arts at California State University and came to Iran to visit her family and to work on a master's degree.
It said Iranian security officials had searched her home and seized property, including films which were part of her project.
Women activists in Iran say they face institutionalized discrimination that makes them second-class citizens in divorce, inheritance, child custody and other aspects of life.
Iran's ruling clerics say women in the country are protected from the sex symbol status they have in the West.
Iran last year detained four Iranians with dual U.S. citizenship on security-related charges, drawing strong protests from Washington. They were later freed on bail.
(Reporting by Hashem Kalantari and Fredrik Dahl; Writing by Fredrik Dahl; Editing by Richard Williams)
