One of the prisoners, Mahbubeh Karami, told her family in a telephone conversation on July 6 that the women are staging the hunger strike to protest their continued detention without charge.
Karami, who is a member of the women's movement Campaign for Equality, was on her way to work on June 24 when police officers detained her while she was riding a bus in Tehran.
Karami’s lawyer tells RFE/RL’s Radio Farda that he has had been unable to meet his client and does not have access to her case.
The reasons behind the detentions are unknown.
Prior to Karami’s arrest, Iranian police briefly detained nine other women’s rights activists who were about to take part in a peaceful seminar to mark the day of Solidarity of Iranian Women.
The seminar, organized by the Campaign for Equality, was due to take place in Rahe Abrisham Gallery in Tehran but security forces prevented the ceremony by arresting the women outside the building and forcing the gallery owner to shut the doors.