Ayatollah Ali Khamenei, Iran's supreme leader, ended his prayer sermon in tears on Friday, invoking the name of a disappeared Shiite prophet to suggest that his government was besieged by forces of evil out to destroy a legitimate Islamic government.
The opposition leader, Mir Hussein Moussavi, in criticizing the government, demanded the kind of justice promised by the Koran and exhorted his followers to take to their rooftops at night to cry out, "Allahu akbar," or "God is great."
In the battle to control Iran's streets, both the government and the opposition are deploying religious symbols and parables to portray themselves as pursing the ideal of a just Islamic state.
That struggle could prove the main fulcrum in the battle for the hearts and minds of most ordinary Iranians, because the Islamic Revolution, since its inception, has painted itself as battling evil. If the government fails the test of being just, not least by using excessive violence against its citizens, it risks letting the opposition wrap itself in the mantle of Islamic virtue.
"If either the reformists or the conservatives can make reference to Islamic values in a way that the majority of citizens understand, they will win," said Mohsen Kadivar, a senior Iranian religious scholar teaching Islamic studies at Duke University.
Perhaps most important, the outcome may determine the support the government enjoys among the ideological zealots who form the backbone of the security forces. Some Iran experts see the level of violence in the week ahead as crucial in the tug of war over Islam.
"What is really smart about Moussavi and his group is that they say they are part of the Islamic Revolution and they want to say 'God is great' and overthrow tyranny," said Said A. Arjomand, a professor of sociology at the State University of New York at Stony Brook. "It is a struggle over the appropriation of the old symbols. If the public says we want Hussein and 'God is great' and then the militias are told to go kill them, that will be a little hard."
The dawn of the Shiite faith can be traced back to the death of Hussein, the Prophet Muhammad's grandson; his killing at the hands of a far larger force in 680 has long infused the faith with a sense of being the underdog. Hence, both sides in Iran portray themselves as ready to be martyrs to their cause — Ayatollah Khamenei suggested it in his sermon, and Mr. Moussavi was quoted as saying that he was also ready to give his life.
The argument on both sides has stayed narrowly within the bounds of Islam, with the opposition even deftly using green, the color of Islam and the family of the prophet, as a subtle symbol that its protests are rooted in the faith. Both sides say they are the true heirs of the revered revolutionary patriarch, Ayatollah Ruhollah Khomeini, in trying to carry out Islamic principles.
In his criticism on Sunday, Mr. Moussavi avoided any direct assault against the supreme leader, instead saying the government cheated on the results of the June 12 presidential election.
"Every Muslim understands that anyone who would lie in this way is not just," said Mr. Kadivar, the Duke professor, who was a senior adviser to the previous reformist president, Mohammad Khatami. "The basic requirement for being the supreme leader is to be just. Justice is a key point in Islamic values."
The argument by Ayatollah Khamenei, laid out in his Friday sermon, is that he is the spiritual guide and therefore challenging him is challenging Islam. In the short term he probably has the more potent argument, analysts said, but sustained violence to subdue demonstrations will work against him.
"Both sides want to paint the other as responsible for the violence," said Mr. Arjomand, adding that the opposition could not label Ayatollah Khamenei a dictator. "They don't want to push it too far; they know they will lose because ultimately Khamenei has the better claim to being Khomeini's heir."
On the other hand, every time anyone inside Iran opens a Web site and sees the images of a teenage girl shot dead in a protest, it chips away at the government's claim to being moral.
"If the movement is successful, they could spread the idea that the regime is evil," said Fatimah Haghighatjoo, a former reformist member of the Iranian Parliament who is now a visiting scholar at the University of Massachusetts in Boston.
In student uprisings against the government in 1999 and 2003, demonstrators challenged the idea of having a supreme leader. It proved relatively easy to crush them, not just because of their small numbers, but because they were challenging the very foundation of the system.
"The people inside Iran are not saying they want regime change. They are saying, 'Where is my vote?' " Ms. Haghighatjoo said. "It is just people coming down into the streets to defend their vote; they can't accuse them of being anti-regime. I don't think the lowest-level Basijis would accept shooting people because they are protesting cheating on the elections."
The Basij — paramilitary, plainclothes vigilantes — is the main force the government uses to try to dispel antigovernment protesters. Thus far, that has been done mostly through beatings, arrests and other intimidation tactics. But the death toll is reportedly 10 to 19 people nationwide.
"In general, the Basij is an ideologically or culturally driven force, but the majority are very committed fundamentalists," said Afshon P. Ostovar, who is writing his doctoral thesis at the University of Michigan about the Iranian security forces.
That ideology may not hold out if the crackdowns start to divide society, however. "You may see the middle ground in the Basij and even the Revolutionary Guards start to question their commitment," Mr. Ostovar said.
This is one reason the suppression has not been as violent as it could be, he added.
Analysts also believe that the government has been holding back because the most senior ayatollahs, who so far have been largely silent about the election, could no longer sit on the sidelines. A few prominent liberal ayatollahs have criticized the election outcome.
But wider bloodshed would likely prompt the most senior conservative clerics in the holy city of Qum, as well as Najaf in Iraq, to weigh in against the government. That would have a significant impact on turning popular opinion toward the opposition.
The strength of the protests is that they have remained within religion, said Roxanne Varzi, an anthropologist at the University of California, Irvine, who has studied the way the government spreads its ideology.
"It was easier to play on the discourse of the infidel versus the righteous citizen," said Ms. Varzi, but the opposition movement adopted the whole Islamic discourse. "It is not meant to be something anti-Islamic, even for those who are secular in their practices. Because they have kept inside that structure, it is hard for the government to justify clamping down on them."