Dozens of Iranian opposition supporters were arrested in Tehran after protesting in support of the recent uprisings in Egypt and Tunisia.
Thousands of people marched to the city’s Azadi Square where they clashed with security forces.
Officers fired tear gas to try and disperse the crowds.
One person was killed and several were wounded, according to the official Fars news agency.
Iranian state television only ran footage of a pro-government demonstration.
It accused opponents of working for the West.
Demonstrators from the two camps clashed later on Monday as tensions ran high.
The march is the first time the Iranian opposition has taken to the streets since December 2009.
Khamenei described the revolts in Egypt and Tunisia as an “Islamic awakening” but his regime has always cracked down on such dissent at home.
British Foreign Secretary William Hague urged Iran to allow people the right to demonstrate.
US Secretary of State Hillary Clinton hailed the “courage” and “aspirations” of the protesters.
Here is a selection of amateur footage:
In Tehran. People are chanting: “I will kill those who kill my brother”, they usually chant this when people are shot. Tear gas is shot. At the end, people are chanting “down with dictator”.