When a pronouncement by a religious scholar in Iran drove Iraq to the brink of civil war last week, there was only one man who could stop it: a 92-year-old Iraqi Shi'ite cleric who proved once again he is the most powerful man in his country.

Grand Ayatollah Ali Al-Sistani said nothing in public about the unrest that erupted on Iraq's streets. But government officials and Shi'ite insiders say it was only Sistani's stance behind the scenes that halted a meltdown.

The story of Iraq's bloodiest week in nearly three years shows the limits of traditional politics in a country where the power to start and stop wars rests with clerics - many with ambiguous ties to Iran, the Shi'ite theocracy next door.

The Iraqis who took to the streets blamed Tehran for whipping up the violence, which began after a cleric based in Iran denounced Iraq's most popular politician, Moqtada al-Sadr, and instructed his own followers - including Sadr himself - to seek guidance from Iran's Supreme Leader.

Sadr's followers tried to storm government buildings. By nightfall they were driving through Baghdad in pickup trucks brandishing machineguns and bazookas.

Armed men believed to be members of pro-Iranian militia opened fire on Sadrist demonstrators who threw stones. At least 30 people were killed.

And then, within 24 hours, it was over as suddenly as it started. Sadr returned to the airwaves and called for calm. His armed supporters and unarmed followers began leaving the streets, the army lifted an overnight curfew and a fragile calm descended upon the capital.

To understand both how the unrest broke out and how it was quelled, Reuters spoke with nearly 20 officials from the Iraqi government, Sadr's movement and rival Shi'ite factions seen as pro-Iranian. Most spoke on condition of anonymity.

Those interviews all pointed to a decisive intervention behind the scenes by Sistani, who has never held formal political office in Iraq but presides as the most influential scholar in its Shi'ite religious centre, Najaf.

According to the officials, Sistani's office ensured Sadr understood that unless Sadr called off the violence by his followers, Sistani would denounce the unrest.

"Sistani sent a message to Sadr, that if he will not stop the violence then Sistani would be forced to release a statement calling for a stopping of fighting - this would have made Sadr look weak, and as if he'd caused bloodshed in Iraq," said an Iraqi government official.

Three Shi'ite figures based in Najaf and close to Sistani would not confirm that Sistani's office sent an explicit message to Sadr. But they said it would have been clear to Sadr that Sistani would soon speak out unless Sadr called off the unrest.

An Iran-aligned official in the region said that if it were not for Sistani's office, "Moqtada al-Sadr would not have held his press conference" that halted the fighting.

'BETRAYAL'

Sistani's intervention may have averted wider bloodshed for now. But it does not solve the problem of maintaining calm in a country where so much power resides outside the political system in the Shi'ite clergy, including among clerics with intimate ties to Iran.

Sistani, who has intervened decisively at crucial moments in Iraq's history since the U.S. invasion that toppled Saddam Hussein, has no obvious successor. Despite his age, little is known publicly about the state of his health.

Meanwhile, many of the most influential Shi'ite figures - including Sadr himself at various points in his career - have studied, lived and worked in Iran, a theocracy which makes no attempt to separate clerical influence from state power.

Last week's violence began after Ayatollah Kadhim al-Haeri, a top ranking Iraqi-born Shi'ite cleric who has lived in Iran for decades, announced he was retiring from public life and shutting down his office due to advanced age. Such a move is practically unknown in the 1,300-year history of Shi'ite Islam, where top clerics are typically revered until death.

Haeri had been anointed as Sadr's movement's spiritual advisor by Sadr's father, himself a revered cleric who was assassinated by Saddam's regime in 1999. In announcing his own resignation, Haeri denounced Sadr for causing rifts among Shi'ites, and called on his own followers to seek future guidance on religious matters from Ayatollah Ali Khamenei - the cleric who also happens to rule the Iranian state.

Sadr made clear in public that he blamed outsiders - implicitly Tehran - for Haeri's intervention: "I don't believe he did this of his own volition," Sadr tweeted.

A senior Baghdad-based member of Sadr's movement said Sadr was furious. "Haeri was Sadr's spiritual guide. Sadr saw it as a betrayal that aimed to rob him of his religious legitimacy as a Shi'ite leader, at a time when he's fighting Iran-backed groups for power."

Sadrist officials in Najaf said the move meant Sadr would have to choose between obeying his spiritual guide Haeri and following Khamenei, or rejecting him and potentially upsetting older figures in his movement who were close to Sadr's father.

Instead, Sadr announced his own withdrawal from politics altogether, a move that spurred his followers onto the street.

The Iranian government and Sadr's office did not immediately respond to request for comment for this story. Haeri's office could not immediately be reached.

Specialists in Shi'ite Islam say Haeri's move to shut his own office and direct his followers to back the Iranian leader would certainly have appeared suspicious in an Iraqi context, where suggestions of Iranian meddling are explosive.

"There's strong reason to believe this was influenced by Iranian pressure - but let's not forget that Haeri has also had disagreements with Sadr in the past," said Marsin Alshammary, a research fellow at the Harvard Kennedy School.

"He directs followers to Khamenei when there's no (religious) need to do so. And it seems unlikely a person in his position would shut down his offices which are probably quite lucrative," she said.

VIOLENCE IS ONE OF THE TOOLS

As gun battles raged in central Baghdad, Sadr stayed silent for nearly 24 hours.

During that time, Shi'ite religious figures across Iraq tried to convince Sadr to stop the violence. They were joined by Shi'ite figures in Iran and Lebanon, according to officials in those countries, who said pressure on Sadr was channelled through Sistani's office in Najaf.

"The Iranians are not intervening directly. They're stung by the backlash against their influence in Iraq and are trying to influence events from a distance," an Iraqi government official said.

Baghdad was calm on Friday, but the deadlock remains.

Sadr insists on new elections, while some Iran-backed groups want to press ahead to form a government. Clashes broke out late in the week in oil-rich southern Iraq.

The government has been largely silent. Prime Minister Mustafa al-Kadhimi said on Tuesday he would step down if violence continued, in a statement made hours after fighting had already stopped.

"Where is the prime minister, the commander-in-chief, in all of this?" said Renad Mansour of the London-based Chatham House think tank. More violence was possible, Mansour said.

"Sadr's main focus is to become the main Shi'ite actor in Iraq, and so he wants to go after his Shi'ite opponents. In Iraq, violence is one of the tools used to compete."