Iranian media say police and demonstrators clashed in Tehran Thursday when activists defied a government warning against demonstrations.
Iranian state-run television says security forces fired tear gas to disperse protesters, who gathered near Tehran University to mark the 10th anniversary of a student uprising.
The station says hundreds of demonstrators gathered near the site, but other news agencies say thousands turned out for the protests.
Witnesses say police fired shots into the air and also used batons and tear gas to hold back protesters. They also say demonstrators chanted "Death to the dictator" and set trash cans on fire.
Witnesses say police detained some of the demonstrators, and they also say members of the pro-government Basij militia were out in force.
Supporters of opposition leader Mir Hossein Mousavi had called for demonstrations to mark Thursday's 10th anniversary of a deadly crackdown against students in Tehran.
However, Iranian authorities had warned that security forces would "smash" any protests. The state-run IRNA news agency quoted Tehran's governor, Morteza Tamadon as saying no government permits had been issued for rallies and there would be a "crushing response" on anyone who defied the protest ban.
Opposition demonstrations broke out following Iran's June 12 presidential vote, which defeated reformist candidate Mir Hossein Mousavi and his supporters say was fraudulent. They have refused to accept official results that show incumbent President Mahmoud Ahmadinejad winning by a landslide.
Iranian police have detained more than 1,000 political activists, journalists, and others who authorities say were linked to post-election demonstrations. Officials say most of them have been released.
On July 9, 1999, security forces raided a dormitory in Tehran, following a peaceful protest. Many students were beaten and arrested. Officials say one student died in the crackdown. Opposition groups say eight people were killed.
Some information for this report was provided by AFP, AP and Reuters.