Clashes have broken out between police and opposition supporters in the Iranian capital, Tehran.
Some reports in Iran say shots were fired by police, but there is no confirmation of this.
Security forces are also said to have used tear gas and batons against the protesters.
Iran’s Revolutionary Guards had warned the opposition not to try to demonstrate against the government during annual anti-US marches.
The official rallies are held to mark the 30th anniversary of the storming of the US embassy.
Tension is still high in Iran after June’s disputed presidential ballot and the government is eager to prevent any revival of demonstrations.Supreme Leader Ayatollah Ali Khamenei recently said it was a crime to question the June 12 vote that secured the re-election of President Mahmoud Ahmadinejad. Anti-Western rallies take place annually outside the US embassy to mark the anniversary of the day in 1979 that the building was seized. The militants held 52 Americans hostage for 444 days.