Hundreds of thousands of people marched on Sunday in Tehran and other cities chanting "Death to America" as Iran marked the 34th anniversary of the Islamic revolution that ousted the US-backed shah.
In the capital, crowds waving Iranian flags and portraits of revolutionary leader Ayatollah Ruhollah Khomeini walked towards the landmark Azadi (Freedom) Square, in a government-sponsored rally which is now a cornerstone of the regime.
Marchers also chanted "Death to Israel" and "Death to America" as they headed for the square, some waving posters of supreme leader Ayatollah Ali Khamenei, where President Mahmoud Ahmadinejad was expected to make an address.
Iran is holding similar rallies nationwide, especially in large provincial capitals such as Mashhad, Isfahan, Shiraz and Kerman.
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At the Tehran rally, foreign media were being closely monitored and allowed to cover the event from officially designated areas only.
The rally marks February 11 when the army declared solidarity with the people, turning its back on shah Mohammad Reza Pahlavi. Ten days beforehand, Khomeini returned in triumph from exile in France to lead the revolutionaries to power.
Tehran is currently under a series of international sanctions aimed at curbing its controversial nuclear programme of uranium enrichment.
World powers and Iran's arch regional foe Israel suspect that Tehran is trying to develop atomic weapons under the cover of its civilian programme, a charge repeatedly and vehemently denied by the Islamic republic.
The sanctions have led to a severe economic crisis, choking Iran's banking system and limiting its oil exports, the country's main foreign revenue earner.
According to a recent survey by the US polling firm Gallup, Iran's nuclear programme is supported by a large majority of its population.