BBC News, Tehran
Iranian woman have increasingly campaigned for fair treatment
Hundreds of civil society activists in Iran have signed an open letter calling for the release of women's rights defender Khadijeh Moghaddam.
Mrs Moghaddam, who is also a campaigner for the environment, was arrested on 8 April and accused of acting against national security.
She is very involved in the drive to gather one million signatures opposing laws that discriminate against women.
The open letter was sent to newspapers and websites across Iran.
Bail for Mrs Moghaddam, 56, was set at more than $100,000 (63,000 euros).
Human rights groups say Mrs Moghaddam was arrested at her home on Tuesday.
Earlier this week Amnesty International said it believed she was being held solely on account of her peaceful activities in support of equal rights for Iranian women.
Among those calling for her release are prominent writers, journalists, lawyers and reformist politicians.
The letter's signatories also include Parvin Ardalan, a founder of the One Million Signatures campaign, who last month was prevented by the Iranian authorities from leaving the country to collect a prestigious human rights prize abroad.
A number of international human rights groups have accused the Iranian authorities of targeting women's rights activists in Iran as part of a wider crackdown on dissenting voices.
They say a number of activists from the One Million Signatures campaign are among those who have been arrested.