TEHRAN, Iran - With titans of the Islamic republic entrenched against each other, crowds of protesters clad in green and black pressed into President Mahmoud Ahmadinejad's home turf yesterday to make the case that he won reelection through massive vote fraud.

The fourth day of demonstrations came as Ayatollah Ali Khamenei, Iran's supreme leader, was expected to address the nation at Friday prayers.

Despite Khamenei's status, analysts said he had little room to maneuver: There appeared to be no constitutional mechanism to end Iran's biggest political challenge in 30 years, and the republic's factional politics have become a bloodsport.

Dressed in black to mourn those killed in recent clashes and green to mark their allegiance to rival candidate Mir Hossein Mousavi, the protesters moved into traditionally conservative southern Tehran.

Many shops were closed. But some workers hung green fabric from balconies and windows or flashed victory signs with their fingers in support of the protesters.

The Guardian Council, the nation's constitutional watchdog, agreed to review specific complaints of Mousavi and two other candidates running against Ahmadinejad but has not broached the widespread belief among Mousavi supporters that the vote was rigged.

Analysts said the constitution provided no means for defusing the crisis.

"I don't think they know what [compromise] means," Anoush Ehteshami, an Iran expert at Durham University in England, said of the hard-line faction now dominating Iran's politics.

The unrest has left at least 12 dead and led to the arrest of government critics. The Association of Human Rights Activists in Iran maintained that at least 32 people were killed, but that number could not be confirmed. Protesters have clashed with pro-government vigilantes from Ansar-e Hezbollah and the Basij militia.

Ahmadinejad appeared to make a slightly conciliatory gesture, clarifying comments from a speech Sunday. According to the Islamic Republic News Agency, he said that his reference to "dirt" was meant to describe alleged vandals and arsonists, not those who voted against him. The statement had enraged many Iranians, who showed up at a rally Monday. Ahmadinejad said the media misinterpreted his comments.

Within the Iranian establishment, moderate and reformist clergy and political figures are joining forces against Ahmadinejad and his hard-line factions.

Authorities are continuing a crackdown on dissidents and intellectuals they suspect are fomenting what they describe as a Western-backed "Green Wave." The Ministry of Intelligence announced that it had "discovered and neutralized" enemy plots "guided by foreigners," according to state television.

Mousavi and his wife, Zahra Rahnavard, arrived at yesterday's rally in two black sport-utility vehicles followed by motorcyclists in green bandannas holding up victory signs, according to witnesses. At Enghelab Square he left the vehicle and spoke to supporters, a witness said. According to news agencies, he criticized state-controlled television as being one-sided in its descriptions of the recent protests and asked people to return to the streets tomorrow.

Khamenei was expected to address the issue of the elections today during a heavily anticipated prayer sermon. Huge crowds figured to attend in a show of support for the cleric's authority, including officials who will fly in from around the country and Basij militias that will be bused in.

But analysts say there are few options for quelling the swell of discontent over Ahmadinejad's election. If Khamenei decides to side with Ahmadinejad, the constitution provides no mechanism to appeal. The supreme leader is considered God's representative on Earth under Iran's political system, which combines elements of a republic with a theocracy.

In the unlikely event he sides with Mousavi and calls for a new election, he would betray his own political base, including the Basiji militiamen and social networks that are his core supporters. Those same networks adore Ahmadinejad.

"I believe he's going to refuse or reject the demand for new elections," said Ali-Reza Nourizadah, an Iran analyst in London. "And after that, well, that's actually the statement for a confrontation."

One possible compromise would be for Ahmadinejad to offer Mousavi a few key cabinet posts. But the factional politics that have bedeviled Iran since the founding of the Islamic republic might prevent that. Few expect Ahmadinejad or other hard-liners who have spent a decade fighting back a reformist tide to compromise. It's not certain that Mousavi, a former prime minister, would accept it, either.

"At this point both sides think they must go ahead as long as it requires," Nourizadeh said.