More than 200 teachers were arrested on Wednesday during a protest outside the parliament in Tehran demanding the release of their colleagues from jail, an activist group said.
Authorities launched a crackdown after over 2,000 teachers from across Iran gathered outside parliament carrying placards and chanting "Free those arrested," the Paris-based National Council of Resistance of Iran (NCRI) said.
Anti-riot police on motorbikes roamed the streets and there was a heavy security presence in metro stations, the group of Iranian pro-democracy activists in exile said in a statement.
"The number of those arrested in today's teachers' protest gathering in Tehran has reached 200," it said.
"The suppressive forces employed provisions to disperse the teachers, including by conducting attacks on the gathering... However, the freedom-loving teachers fought (them) off to again form their gathering."
Protesters had gathered demanding the release of political activists such as Esmael Abdi, the head of the teachers' union, who are "imprisoned for their defence of teachers' rights," the group said.
The NCRI is a coalition of Iranian opposition groups, including the popular People's Mojahedin Organization of Iran, which is considered a terrorist organisation by the US and EU.
There was no immediate comment from Iranian authorities.