BBC News - Iran police fire tear gas at opposition rally in Tehran
In their first major show of dissent since December 2009, thousands of opposition supporters defied the government ban and gathered at Tehran's Azadi Square on Monday. They chanted "Death to dictators".
Riot police and plain-clothes police backed by the Republican Guards used tear gas to disperse the protesters, he adds.
Other witnesses said police also fired paintball guns at demonstrators.
The fiercest clashes were reported on Azerbaijan Street, close to Azadi Square, and one witness said a number of ambulances had come and gone.