Tehran  - Police and protestors criticizing Iranian President Mahmoud Ahmadinejad's government clashed Wednesday in Tehran as demonstrators shouted slogans such as "Death to the dictator" and authorities responded with tear gas, witnesses said.

The clashes occurred on a square near the US embassy, where the main state-organized anti-US rally is to be held Wednesday in observance of the Day of National Confrontation against World Imperialism. The day is marked on November 4 every year, the anniversary of the 1979 occupation of the US embassy in Iran in support of the Iranian Revolution.

Supporters of the opposition - mainly members of the Green Movement, which backs opposition leader and this year's failed presidential candidate Mir-Hossein Moussavi - were also using the 30th anniversary to protest against Ahmadinejad.

Iranian police and security were deployed a large numbers in downtown Tehran ahead of the anti-government protests.

Volunteer pro-Ahmadinejad militia forces joined the government forces in deploying in alleys to confront the anti-government protestors, who were expected to number in the tens of thousands. (dpa)