A prominent Iranian lawyer who defended women arrested when they defied Iran’s head-covering rule has been convicted of security-related crimes in a secret trial and could face a “very lengthy sentence,” a human-rights monitoring group reported Wednesday.

The group, the Center for Human Rights in Iran, said it had learned of the conviction of the lawyer, Nasrin Sotoudeh, from her husband. She was seized at her home by security agents last June and placed in Evin Prison in Tehran.

Ms. Sotoudeh, 55, who has been in and out of Iranian prisons several times, is an international symbol of defiance to the limits on personal and political freedoms imposed by the Islamic Republic’s religious hierarchy. She won Europe’s most prestigious human rights award, the Sakharov Prize for Freedom of Thought, in 2012.

The Iranian authorities have never specified why they seized her in June, but at the time Ms. Sotoudeh was defending women arrested when they removed their hijabs, or Islamic head scarves, in public protests.