BEIRUT (Reuters) - Iran's rulers are tensing for a new trial of strength with protesters on Sunday, when an emotive Shi'ite ritual coincides with seventh-day mourning for a defiant ayatollah who was once a heartbeat from the pinnacle of power.
A loose reformist opposition movement that swelled after a disputed presidential election in June is exploiting the rhythms of the Islamic Republic's political-religious calendar to stage rolling protests and sustain the momentum of its struggle.
On Wednesday, security forces fought with protesters marking the third day after the death of Grand Ayatollah Hossein Ali Montazeri, the reformists' spiritual patron, in the cities of Isfahan and Najafabad, his birthplace, a reformist website said.
The outcome of Iran's internal upheaval is uncertain -- and restrictions on independent reporting hamper any assessment of what is unfolding in a complex society -- but it may have set a new clock ticking, alongside the nuclear one that drives Western urgency in dealings with the world's fifth biggest oil exporter.
"It is going to get messier," said Ali Ansari, a professor at Scotland's St. Andrews University.
"It's wishful thinking when people say nothing is going to happen or there is going to be a smooth 'velvet revolution'. It is going to get very violent."
More protests may flare on Sunday when Montazeri's followers observe the traditional seventh day of mourning -- which falls on Ashura, the climax of passionate Shi'ite commemorations of Hussein, martyred grandson of the Prophet Mohammad.
Montazeri, aged 87 when he died at the weekend, had attacked Iran's "dictatorship" for its handling of post-election unrest.
A stalwart of the 1979 Islamic revolution, he had been named to succeed its founding father, Ayatollah Ruhollah Khomeini, until he confronted the leader over mass killings of prisoners.
"Apart from his clerical status, what made Montazeri unique was his integrity," said Karim Sadjadpour, an associate at the Carnegie Endowment for International Peace in Washington.
"Had he simply turned a blind eye to the regime's human rights abuses during the 1980s, he would have succeeded Khomeini as Supreme Leader. Instead he spent the latter years of his life under house arrest," he said of the cleric who lived in Qom.
Montazeri's tumultuous funeral on Monday in the shrine city drew huge crowds, some of them chanting anti-government slogans and scuffling with hardline Basij religious militiamen.
In an apparent warning to other dissenting clerics, Basijis attacked the house of Grand Ayatollah Yusuf Sanei, a moderate voice for reform, in Qom on Tuesday, a reformist website said.
A hardline cleric who is an aide to Supreme Leader Ayatollah Ali Khamenei accused the opposition of using Montazeri's funeral to stir "chaos" to undermine the Iranian leadership.
Mojtaba Zolnour, a representative of Khamenei with the powerful Revolutionary Guards, also criticized Montazeri for "misusing his power" when he was an aide to Khomeini.
"The establishment has enough evidence for confronting the leaders of sedition," Zolnour was quoted by the semi-official Fars news agency as saying. "Don't think that the reason we do not confront them is because we are weak ... Nothing will happen even if we arrest all three leaders of sedition."
He appeared to be trying to dispel the notion that the arrest or other means of "confronting" the three -- Mirhossein Mousavi and his reformist allies Mehdi Karoubi and former President Mohammad Khatami -- would spark a furious reaction on the streets.
Zolnour's remarks might also reflect a fearful logic that makes compromise with the opposition almost unthinkable.
As the ruling system grapples with its severest challenge in 30 years, the men in power are determined not to repeat the "weakness" that they believe sealed the shah's fate in 1979.
Many Iran analysts abroad say the disparate opposition movement has shown its staying power in a reshaped political landscape -- a view questioned by Mohammad Marandi, a professor of North American Studies at Tehran University.
"It will continue for a few more months, maybe a year, or less than a year," he told Reuters. "It has already died down."
Mousavi, who blamed fraud for his election loss to President Mahmoud Ahmadinejad, has alienated traditional reformists within the establishment, Marandi said, leaving his "Green Movement" with only a rump of hardcore supporters.
For now, many Iranians still seem ready periodically to don the emblematic green adopted by Mousavi supporters to defy the government in the streets, despite crackdowns and arrests.
"The opposition doesn't appear willing to back down any time soon," said Alireza Nader, a RAND Corporation analyst.
"It is the Iranian population, represented in parts by the 'Green Movement', and at times without a visible leader, that will keep the momentum alive," he argued.
Ansari said initial violence against protesters had turned an election dispute into a more fundamental conflict, albeit one still aimed at reforming, not toppling, the Islamic Republic.
"People are very unforgiving," he said, questioning how long Khamenei could survive popular pressure for change.
The aftermath of the election divided the nation, including Shi'ite clerics, among whom Montazeri was the most senior, but Carnegie's Sadjadpour suggested the clergy no longer provided the main bedrock of support for the ruling establishment.
"Over the last decade the Revolutionary Guards have eclipsed the clergy in terms of their political and economic power.
"Widening cleavages among clerical elites is worrisome but manageable for the regime, but if the Revolutionary Guards start splintering it could prove fatal," Sadjadpour said.