Opposition and pro-government Iranians are preparing to take to the streets of Iran on Thursday to mark the 31st anniversary of the Islamic Revolution, in a political and ideological showdown set to shape the future of Iran.

Supreme religious leader Ayatollah Seyyed Ali Khamenei has promised that the West and Iran’s enemies would be “stupefied” by an unprecedented show of support for the regime.

But every senior leader of Iran’s opposition – the so-called Green Movement that grew out of disputed elections last June – has also called for huge turnout on a day that has always been considered by many Iranians as more a nationalistic than pro-regime event.

“It’s going to be a big show of force, a big competition between the two sides over who can rule this day – and who can lay claim to the legacy of the revolution,” says Ahmad Sadri, an Iran specialist at Lake Forest College in Illinois.

At issue are fundamental questions that have been fought over in the streets, in the prisons, and from the pulpits of Iran for eight months. At stake is who are the true heirs of Iran's revolution and its first promises of “Independence, freedom, [and] Islamic Republic.”

Weeks of violence were sparked last summer by the officially declared election landslide for incumbent hardline President Mahmoud Ahmadinejad, which many Iranians and analysts say was an impossible result achieved through fraud.

Protests since then have veered increasingly toward challenging Iran’s entire Islamic system led by Ayatollah Khamenei. But opposition leaders such as former prime minister and presidential candidate Mir Hossein Mousavi have called for greater moderation from supporters on Thursday to avoid provoking violence.

Violence in store?

At least eight were killed in clashes – with some reports saying more than 30 – during religious commemorations at the end of December.

“Everybody knows that people in the streets, many of them, are not at all content with the present regime,” says Mr. Sadri. “But I think there is a chance that they would heed this particular request as a strategic move to deny the government the excuse to go after them with the brutality of they have shown in the past.”

As early as Tuesday night, security forces set up nighttime checkpoints in some districts in Tehran. “They are going to intimidate people,” said one close observer in the capital who asked not to be named. “It’s going to be a decisive showdown.”

Authorities have warned repeatedly in recent weeks that they would not permit the anniversary rally to be hijacked by Green Movement protesters, as has taken place on a number of previous calendar dates important for the regime.

The opposition are “both hopeful and anxious, and still angry,” says the analyst about the state of mind of the opposition. “They are hopeful that they can force the government to back down a bit, but they are not sure about that. They don’t see violence, but they are confronted with violence.”

Senior police officers have vowed that events on Thursday would prove the final nail in the coffin for opponents of the regime, who have been branded Mohareb, or “enemies of God.” Scores have died in protests and thousands have been arrested. Iran's detention centers have consistently yielded stories of abuse and even rape.

Yet tough efforts by security forces – both official and unofficial regime enforcers – have failed to stop the protest movement.

The violence that accompanied Ashura, which has historically lionized martyrdom and came this year in late December, “was a serious blow – they want to reverse it,” says the analyst in Tehran. “On previous occasions they have been defeated in their plans [to control the streets] by some unpredictable forces and events. The huge number of people can change everything.”

Both sides of Iran’s political divide have been preparing for the anniversary, which traditionally includes a speech from the sitting president and state TV broadcasts showing huge crowds across the country shouting “Death to America” and “Death to Israel” while they burn the flags of these perennial regime enemies.

One Revolutionary Guard commander said that any protest would be met with force. "The security forces will be after maintaining the safety of the demonstrations and will fiercely confront anyone who might want to fall out of line with the revolution's caravan," said Hossein Hamedani, according to ISNA news agency. The revolution anniversary, he added, "belongs to all 70 million Iranians and we will not let anyone confiscate it for the sake of a certain group."

Protesters prepare

One list of tips for protesters circulating on the Internet gave practical advice, such as carrying a napkin wet with vinegar to “fight the effects of tear gas,” and those with medical experience to carry small first aid kits. The list asked protesters not to carry weapons such as knives, but called on people living along protest and rally routes to stash sacks full of stones for throwing.

“We must make it as difficult as possible for Ahmadinejad to give a speech on the main podium,” the list suggested, adding that “creating disturbances around the square and affecting Ahmadinejad’s psychological preparation for his speech should be our first priority.”

But Khamenei warned in recent days against giving heed to any dissenters: "Whenever the great masses of the people – all over Iran – feel danger or serious enmity towards the revolution, they go to the streets without being called upon," he said, according to a translation by IranTracker.org. "Of course, ever since the beginning of the victory of the Islamic revolution, there were some who were opposed to this regime and desired return to hegemony of the United States. Today they are both inside and outside the country."

In years past on the anniversary, this reporter has heard a cross-section of views about the Islamic Republic, both from die-hard supporters of the regime and those who believed that its promises of social justice, economic improvement, and freedom had yet to be met after three decades.

Mr. Mousavi spoke recently about the need to change the Constitution – and democratize it.

“The basic agenda of the reform movement has been … to make the power of the Supreme Leader into a ceremonial power, rather than a real source of power,” says Sadri.

But that is a counter-revolutionary view to hard-liners in Iran, who know they are fighting for their political lives against fellow Iranians they believe to be “enemies.”

“The trick for reformers is how to neutralize them. Because not only are [hardliners] in power, but they have a base. They have the Basij [militia] and Revolutionary Guard…and also people who sincerely believe that this is Islamic government,” adds Sadri.

“Even if the Green Movement succeeds … if you don’t include [the hard-line base], they can turn into some kind of Baath Party, and blow up things the way they are doing in Iraq. Even if we win … they can shovel sand into the gears of this democratic movement with obstructionism, with terrorism.”

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