Thousands of opposition supporters have clashed with security forces during a government-sponsored rally in Tehran.
Iran's reformists had been warned not to try to turn the pro-Palestinian Quds (Jerusalem) Day marches into anti-government protests.
Reports say opposition leaders Mir-Hossein Mousavi and former President Mohammad Khatami were attacked.
The opposition has been banned from holding rallies since a disputed presidential election on 20 June.
At a prayer ceremony after the rally, President Mahmoud Ahmadinejad delivered a speech in which he repeated his view that the Nazi Holocaust was a myth.
The Quds Day rallies are held annually on the last Friday of the Muslim fasting month of Ramadan.
The BBC reports it was the first time in two months that opposition protesters have been on the streets in numbers.