Enduring America is live-blogging:
1630 GMT: The BBC's account of events in Tehran reports use of tear gas by police against thousands of protesters. A BBC producer, affected by the gas, described central Tehran as "total chaos" with "severe clashes" and many arrests. ...
1500 GMT: ... Reports of violence are now widespread, with comparisons with the march of 15 June 2009 flowing in. BBC Persian is quoting a "secure and certain source" that tear and pepper gas deployed in Imam Hossein Square. Police deployed everywhere along Enghelab Avenue, blocking traffic. People are trying to get to Azadi Square on foot, with security forces getting more aggressive as marchers near the square. ...
1343 GMT: According to CNN's Reza Sayah, there have been violent clashes in front of Tehran University, where some protesters have been taken away on motorcycle. There are also many protesters clashing with security forces at Iman Hossein Square, where people are chanting "Death to the Dictator, Death to Khamenei."
More footage after the jump:
An Iranian reader translates the chant:
Mubarak ... Ben Ali ... Now it’s your turn, Seid Ali (first name of the Supreme Leader)