Narges Hosseini was arrested within 10 minutes of removing her hijab along with two people filming her, Iran’s foremost human rights lawyer, Nasrin Sotoudeh, told The Guardian.
Ms Hosseini was pictured wearing a green wristband in an apparent reference to the Iranian Green Movement which rose in protest after the 2009 presidential election.
“Her message is clear, girls and women are fed up with forced [hijab],” Ms Sotoudeh wrote on Facebook. ”Let women decide themselves about their own body.”
Iran protests – in pictures
While it is unclear where the images shared on social media were taken, women who fail to wear a headscarf in the Islamic Republic can be jailed for up to two months or fined 500,000 rials (£9.50).
The Muslim dress code, in place since the 1979 revolution, considers veiling obligatory for any female above 13 and says they should cover themselves from head to toe while disavowing any figure-hugging dress.
The announcement suggested an easing, called for by the young and reform-minded Iranians who helped re-elect Hassan Rouhani as president earlier this year.
Many Islamists believe hijab should not be imposed by the law, but consider it a religious obligation for every Muslim woman.
Both Mr Rouhani and Iran’s Supreme Leader, Ayatollah Ali Khamenei, support a softer attitude towards women with improper hijab, although hard-liners opposed to easing such rules still dominate Iran’s security forces and the judiciary.