Iran's opposition on Sunday seized upon the death of one of the Islamic republic's founding fathers -- a revered ayatollah who was also a fierce critic of the nation's leadership -- to take to the streets in mourning.
Fearing that mourners could quickly turn into antigovernment protesters, Iranian authorities tightened security across the country. In Tehran, crowds held up pictures of the dead cleric and chanted, "This is the month of blood, the regime is coming down," according to eyewitnesses and videos posted on YouTube.
But the death of Grand Ayatollah Hossein Ali Montazeri, who passed away in his sleep, was only one of two surprises to shake Iran over the weekend.
Hours earlier, on Saturday, military prosecutors alleged that prison guards tortured to death at least three student protesters in July, contradicting months of denials by top leaders. The reversal is one of the biggest blows to Tehran's credibility since government protests first erupted six months ago.
Either development, by itself, would provide a rallying point for the opposition, which claims last summer's presidential election was a fraud and is demanding a political overhaul. Together, they represent the widening array of challenges facing the Iranian regime.
The murder allegations center on Tehran's notorious Kahrizak prison, where three protesters died in July. A young doctor who claimed to have evidence of torture at the prison, Ramin Pourandarjani, later himself died under mysterious circumstances. In recent weeks Dr. Pourandarjani, who was the subject of a page-one article in The Wall Street Journal on Saturday, has emerged as a martyr figure for the opposition.
Complicating matters, the weekend's events coincide with the 10-day Muharram religious holiday, during which Shiite Muslims traditionally hold emotionally charged street processions to honor a revered Shiite saint. The opposition already had vowed to mark this year's ceremonies with massive daily protests against the government.
Iran's leadership maintains a firm grip on power and quickly moved to assert its control, using tools that have proved effective in the past to tamp down unrest. Prominent opposition figures and activists reported receiving threatening phone calls from security agents on Sunday, warning them against attending Monday's funeral, according to the International Campaign for Human Rights in Iran. Five prominent opposition figures were reportedly arrested on the road to the holy city of Qom, where the ayatollah's funeral service was planned.
On Sunday, as news spread of the death of Ayatollah Montazeri, 87 years old, spontaneous protests erupted in several Iranian cities and at university campuses, according to video circulating on the Internet Sunday afternoon. In one clip from Najafabad, Mr. Montazeri's home town, crowds chanted, "Our green Montazeri, congratulations on your freedom." Green is the adopted color of the Iranian opposition movement.
At several Tehran universities, professors canceled classes. Students staged sit-ins and marches, reciting verses from the Quran and chanting, "It's a day of mourning in Iran, the Green people of Iran are in mourning." Iran's main student-activist group called on students across the nation to take to the streets Monday in a sign of respect for Ayatollah Montazeri.
The family of Ayatollah Montazeri said it would hold the funeral Monday in Qom. By midday Sunday, opposition Web sites were calling on supporters to join them for Ayatollah Montazeri's funeral.
His death provides an opportunity for Iran's dissident clergy members, who have mostly remained on the sidelines, to publicly show support for the opposition. This is significant because, in Iran, political movements have little chance to succeed without backing from the clergy.
On Sunday, Iranian security forces took positions around Ayatollah Montazeri's house, monitoring the parade of visitors, according to his family. "Our house has been packed, people are coming and going, including many of Qom's senior clerics," said grandson Nasser Montazeri, reached by phone in Qom.
For the government, the risk is that mourning processions become political marches.
"The emotional and political impact of Ayatollah Montazeri's death could morph into a widespread flame that the government cannot contain," said Mohamad Javad Akbarein, a religious scholar and former student of the ayatollah's.
This year's antigovernment protests, the biggest since the Islamic revolution more than 30 years ago, are at a crucial point. In recent months, demonstrations have evolved from protests against the handling of the election to denunciations of the Islamic regime and its Supreme Leader Ayatollah Ali Khamenei.
Government-controlled media played down the news of Ayatollah Montazeri's death. Supreme Leader Khamenei offered his condolences, calling him "a well-versed jurist and a prominent master."
Ayatollah Montazeri, frail in recent years, was once in line to succeed Ayatollah Ruhollah Khomeini, the founder of the Islamic Republic, as Supreme Leader. But he and Mr. Khomeini fell out in the late 1980s.
Ayatollah Montazeri accused Ayatollah Khamenei of creating a dictatorship in the name of Islam. Ayatollah Montazeri was placed under house arrest from 1997 to 2003. In time, he gained a large spiritual following because of his advocacy of reform inside the Islamic Republic and his calls for more democracy.
His lofty standing in the clerical establishment -- he outranked even Ayatollah Khamenei as an Islamic scholar -- helped to protect him over the years as he criticized the government. In the past six months, Ayatollah Montazeri's role grew more prominent, dovetailing with demands by protesters who rallied against what they saw as a stolen election in June by President Mahmoud Ahmadinejad.
The ayatollah aligned with the so-called Green Movement, led by opposition candidates who lost to Mr. Ahmadinejad. He harshly criticized Iran's rulers, at one point calling the Ahmadinejad government "illegitimate."
In one of his many broadsides, Ayatollah Montazeri warned Ayatollah Khamenei that merely closing the Kahrizak prison -- site of the alleged torture deaths in July -- was an attempt to "fool people. You can't blame all these sins on a building."
"The people of Iran are not stupid, and if those responsible are not persecuted, people will not remain silent," Ayatollah Montazeri said in a July 29 statement, the day after the prison was shut.
On Saturday, Iranian military prosecutors issued charges against 12 officials at the prison. That action directly challenges the account of prison deaths provided so far by the government of Mr. Ahmadinejad.
The charges underscore what appeared to be a widening rift in the government about how to deal with the protest movement.
Hard-line elements aligned with Mr. Ahmadinejad, including the Revolutionary Guard Corps and the Basij militia, have spearheaded a broad and sometimes violent crackdown on protesters. More moderate elements, including many members of parliament, have insisted on probing alleged abuses in that crackdown.
Ayatollah Khamenei, who has the final say in all matters of state, appears to waver between the two. While he has backed Mr. Ahmadinejad and demanded protesters abandon the streets, he also shut down Kahrizak prison. He would also have approved Saturday's announcement of the charges against the prison staff.
Prosecutors said three prison officials, all from Iran's armed forces, had been charged with first-degree murder. Another nine military officials at the detention center face other, unspecified criminal charges. Authorities didn't name the suspects.
Kahrizak prison was a main depository for prisoners rounded up in the unrest that followed the June elections. At the height of the unrest, some 140 protesters were held there. Three young men died in custody, one of them the son of a top conservative politician.
Allegations of torture there crystallized public outrage, as opposition Web sites published horrifying claims by prisoners of filthy conditions, blood-stained walls and routine beatings in underground holding cells.
Mr. Ahmadinejad's government vehemently denied any abuse took place. Senior security and police officials made multiple statements blaming the deaths in Kahrizak on meningitis. The victims' parents said their sons' bodies had visible signs of torture and beating.
In a statement Saturday posted by official Iranian news agencies, military prosecutors said: "The coroner's office has rejected that meningitis was the cause of the deaths and has confirmed the existence of signs of repeated beatings on the bodies and has declared that the wounds inflicted were the cause of the deaths."
Dr. Pourandarjani, the young doctor who died recently under mysterious circumstances, had seen the bodies of the three victims and signed their death certificates. According to his family, he testified to a parliamentary committee that the prisoners had died of severe blows to their heads and bodies.
Saturday's statement by prosecutors doesn't mention Dr. Pourandarjani. But it does say that they based their conclusions on medical reports of detainees and witness testimonies made to the parliamentary committee. Dr. Pourandarjani is believed to have been the only person in a position to provide these.