The women had gathered outside a court to protest the trial of four women's rights advocates charged in connection with a peaceful protest last June against discriminatory laws.
Amnesty's researcher on Iran, Drewery Dyke, told RFE/RL the latest arrests may be an attempt to prevent activists from marking International Women's Day on March 8.
"Rather than arresting peaceful demonstrators, the Iranian authorities should really be taking women's demands seriously, there's a serious and genuine demand for equality, equality before the law," Dyke said.
The Center of Human Rights Defenders, the Tehran-based rights group founded by Nobel Peace Prize winner Shirin Ebad, has also called on the Iranian authorities to release the women.