Tens of thousands of Iranians rallied on Monday to mark the 29th anniversary of the Islamic revolution, in a show of defiance of Western pressure on Iran to suspend its disputed nuclear program.

Iran will not back down "one iota" in its nuclear dispute with Western powers, President Mahmoud Ahmadinejad told the crowd which had gathered in Tehran to celebrate revolution that toppled the U.S.-backed shah.

"They should know that the Iranian nation will not retreat one iota from its nuclear rights. They ... cannot do anything except to play with papers and make propaganda," Ahmadinejad said, referring to world powers' discussions of a third U.N. Security Council resolution against Tehran over nuclear work the West fears has military aims, though Tehran denies this.

Almost three decades after the revolution, women in head-to-toe black chador and men shouted "Death to America", the ritual chant of the 1979 revolution that swept the Shi'ite Muslim clergy to power.

Iranian officials had called on people to turn out in large numbers to show their unity in the face of Western pressure. State television broadcast live footage of major rallies held in other cities.

The West fears Tehran is covertly trying to obtain nuclear bombs. The Islam Republic says it needs nuclear technology to meet its booming electricity demand.

"With this rally, Western countries will understand that they cannot impose their will on us and they see that we back our leader and government," Ali Salmani, 17, told Reuters in Tehran.

Tehran's defiance of a U.N. Security Council demand that it suspend uranium enrichment, which can have both civilian and military uses, has led to two rounds of sanctions. World powers have drafted broader measures for a third resolution.

"America should understand and believe that the Iranian nation will not back down from its rights," said another demonstrator, Leila Jafari.

Iran and the United States have not had diplomatic ties since shortly after the revolution, when hard-line students seized the U.S. embassy and held 52 Americans hostage for 444 days.