Women who sought to attend the World League match in June were reportedly turned away from the stadium. Female photographers inside the complex were ordered to leave though none were arrested.
Ghavami was held for a few hours and then released but she was detained again a few days later and transferred to Tehran’s notorious Evin jail, which is known for holding political prisoners and journalists.
She began a hunger strike earlier this month over her detention, Amnesty says.
Her brother, 28-year-old Iman Ghavami, said she rang her family in tears saying she had been put in solitary confinement for 41 days.
“The family can barely hold themselves together,” he told ITV News.
“They are torn apart – not just my parents but my grandparents, my uncles, everybody.”
A Facebook campaign to free her has started, garnering almost 9,000 “likes” and lead to protests at other Iranian volleyball matches.
Iran's judiciary spokesman, Gholam Hossein Mohseni Ejehi, has criticized reports linking Ghavami's arrest to volleyball, saying last month: "Her case has nothing to do with sports."
