They were protesting laws that discriminate against women. From the BBC, with thanks to Anna:
Iran’s authorities have arrested more than 25 women activists protesting outside a courthouse in Tehran.
The protesters were showing solidarity with five women on trial for organising a protest last June against laws they say discriminate against women.
The five have been charged with endangering national security, propaganda against the state and taking part in an illegal gathering.
US pressure group, Human Rights Watch, has urged an end to the prosecution.
It said the women had been exercising their right to freedom of peaceful assembly.
The five are organisers of a demonstration last June which was violently broken up by the police and led to the arrest of 70 people, many of them innocent bystanders.