Iranian riot police beat protesters and fired tear gas as violence erupted in Tehran today when thousands of members of the opposition movement took to the streets in open defiance of the country's supreme leader.

Thousands of plain clothes and uniformed security officials blockaded Tehran's main throughfares but it seems many of the demonstrators who had previously turned out in their hundreds of thousands stayed away, for fear of official reprisals.

The crackdown on supporters of the reformist presidential candidate Mir Hossein Mousavi confirmed fears that authorities would carry out their threat to suppress protests in the aftermath of the disputed presidential election.

Eyewitnesses in Revolution Square reported around 20,000 riot police, made up of Basiji militiamen and soldiers, and armed with rifles, tear gas and water cannons, far outnumbering the hardcore of around 3,000 opposition members who turned out today.