Iranian police have used water cannon, batons, tear gas and live rounds to break up protests, according to witnesses in Tehran.
The demonstrators had defied a ban by the supreme leader Ayatollah Khamenei to protest about the disputed presidential election.
Thousands of police officers blocked off Enghelab Square near Tehran University.
The protesters had gathered in streets near the square, where they were chanting, throwing stones and setting rubbish bins alight.
The BBC's Julian Marshall presents the latest for Newshour.
First broadcast 20 June 2009