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This year's February 11 celebrations come as the simmering dispute with world powers over Iran's nuclear programme approaches boiling point after the Islamic republic on Tuesday defiantly began work on high-enriched uranium.
Celebrations to mark the day the US-backed shah fell in 1979 have been traditionally festive, and an opportunity for Iranian leaders to showcase popular support for the establishment.
But this year, opposition groups -- led by some of the founding fathers of the Islamic republic -- look set to hijack the national day as they continue to reject the legitimacy of President Mahmoud Ahmadinejad's government.
If the opposition does succeed in mounting protests, this would be highly symbolic given the anniversary's historic significance.
The elite Revolutionary Guards and police are leaving no stone unturned in their bid to prevent protests which, since they first erupted last June, have threatened the very pillars of the Islamic regime and split the senior clergy.
"If anyone wants to disrupt this glorious ceremony, they will be confronted by people and we too are fully prepared," police chief Esmail Ahmadi Moghaddam warned on Tuesday.
Several people who had been planning to protest are already in custody, he added.
An official coordinating the media said that foreign media have been banned from covering the traditional street marches marking the anniversary and were restricted to reporting only on the speech of Ahmadinejad at the historic Azadi (Freedom) Square.
Ahmadinejad's disputed re-election last June plunged the Islamic republic into one of its worst ever political crises, with the opposition refusing to take the fight off the streets despite often deadly crackdowns.
Most recently, eight people were killed on the Shiite holy day of Ashura on December 27 and hundreds were jailed as authorities battled protesters they accuse of seeking to topple the regime and siding with Iran's enemies abroad.
Iran's all-powerful supreme leader Ayatollah Ali Khamenei says he wants Thursday's celebration to be a show of unity and to deliver a stunning "punch" to "arrogant" powers.
Khamenei -- the commander-in-chief who has the final say on all key national issues -- has openly sided with Ahmadinejad, dismissed allegations of fraud in the election, blamed the West for the post-poll unrest and slammed continuing dissent as "sedition".
"The most important aim of the sedition after the election was to create a rift within the Iranian nation, but it was unable to do so and our nation's unity remained a thorn in its eyes," he said on Monday.
The opposition is led by former stalwarts of the Islamic republic, including one-time premier Mir Hossein Mousavi, who says the 1979 revolution failed because the shah-era "roots of tyranny and dictatorship" still exist.
"Dictatorship in the name of religion is the worst kind," he said on February 2.
Mousavi and Mehdi Karroubi, the other main opposition leader, have urged a mass turnout by their supporters in what is known as the "Green Movement," but also urged them to show restraint.
"I feel we have to participate while maintaining the collective spirit as well as our identity and leave an impression," Mousavi said on Monday. "Anger and bitterness should not take our control away."
Karroubi, a reformist former speaker of parliament, said: "Let us all together take part in the anniversary rally calmly and firmly ... with patience and without verbal and physical violence."
The deadly Ashura protests saw Mousavi's nephew shot dead, and in January Iran executed two men for seeking to overthrow the regime and has said nine protesters were on death row on similar charges.
Ahead of the anniversary, Internet connections slowed to a crawl and text messaging services were disrupted, with the government blaming technical glitches.
In the past the opposition has exploited the Internet and SMS messaging to organise rallies and spread news and pictures of protests. Foreign media are banned from covering opposition demonstrations.
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