By the standards of Iraq and its Shiite Muslim majority, Sayyid Muqtada Sadr is a blue blood.

He wears a black turban, signifying his privileged descent from the prophet Muhammad. For a century, his family has given Iraq its most revered clergy, men whose very word, blessed by God, goes unquestioned by their followers. Like a badge of honor, he bears the deep scars of ousted president Saddam Hussein's government, which assassinated his father and two brothers in 1999 in Najaf, one of the most sacred cities in Shiite Islam.

Now, by birth and choice, the 30-year-old Sadr, his hands soft from a life of religious study, has inherited his family's mantle of leadership.

In the void left by the precipitous fall of Hussein's government after a U.S.-led invasion, Sadr and his followers have overseen checkpoints to end looting and moved, with the force of arms and power of persuasion, to restore authority in the streets. They have kept a distance from U.S. forces, suspicious of their motives. Sadr and his men are cognizant that their authority derives from their independence. With little hesitation, Sadr has reached out to Iraq's powerful tribes for support and rallied his followers from the pulpit of Friday sermons.

In words lacking the usual subtlety of religious discourse, Sadr's message is clear: He is both a political and religious leader, carrying the still-resonant banner of the Sadr name. The future of Iraq, he insists, is in the hands of the Shiite majority he hopes to represent.

"I accept the burden and the responsibility," he said in a rare interview this week while in hiding here, fearful of conflicts with others in the Shiite community. "We are with God and God is with us."

Sadr and other clerics stand at the center of the most decisive moment for Shiite Muslims in Iraq's modern history. It is a revival from both the streets and the seminaries that will most likely shape the destiny of a postwar Iraq.

In the streets, the end of Hussein's rule has unleashed a sweeping and boisterous celebration of faith, from Baghdad to Basra, as Shiites embrace traditions repressed for decades. In politics, the prominence of clergy -- the major institution to survive the repression of Hussein's powerful Baath Party -- has signaled that in coming years, power may be reflected through a religious prism. And for Shiite populations abroad -- including in Iran -- the community's newfound freedom may reestablish the holy cities of Najaf and Karbala as centers of religion and politics, recasting an arc of Shiite activism that began with the 1979 Iranian Revolution.

But the community is already struggling with the challenges that will deepen as a new government is formulated. How will they interact with a United States that has done little to engage them? Will they relinquish the power they have seized to a more representative government, one that also includes Iraq's Sunni Muslims, Christians and Kurds? And how will they reconcile the deepening rivalries of personality and vision within the community that are already tearing away at the unity that the clergy so desire?

Some among Iraq's minority Sunni and Christian communities are gloomy, predicting sectarian strife reminiscent of Lebanon's civil war. They predict the United States will never accept a Shiite government, much less a religious one -- a concern shared by Sadr and many Shiite leaders. Others are more optimistic, hopeful that Iraq's diversity will temper the Shiite community's demands and that its moderation, so far, is a signal of intentions.

"Iraq could become an inspiration to the Shiite world," said Wamid Nadhme, a political science professor at Baghdad University.

Until then, much of Iraq, anxious and uneasy, is watching the writing on the wall -- graffiti that has exploded across Baghdad and other cities. The sentiments in those slogans chart the emergence, concerns and ambitions of the country's resurgent majority.

'The Tyrant Is Gone'

On the walls of Najaf, Karbala and Baghdad, the slogan was simple. The tyrant Saddam. The tyranny has ended.

Within hours of Hussein's fall, hundreds of Shiite Muslims poured into the Kadhimiya shrine in Baghdad in demonstrations that ballooned into thousands. In chants and banners, the symbolism was unmistakable -- a reclamation of a 1,300-year-old faith relentlessly repressed by Hussein since the 1970s.

"The oppression is gone, however long it took," the crowd chanted, louder as they approached the shrine. "The tyrant is gone."

They held the green flags of Imam Ali, the prophet Muhammad's son-in-law, who Shiites believe was his rightful heir. The 7th century dispute over leadership, deepened by centuries of disenfranchisement and discord, has formed the lasting division of Islam into its Sunni and Shiite branches. Others carried the black banner of Hussein, Ali's son who -- outnumbered, betrayed and deprived of water -- was martyred in a battle in 680 at Karbala, near the Euphrates River. He was decapitated, his head carried away on a stake. Yazid, the caliph blamed for his death, is known to Shiites as the tyrant.

The celebrations are building toward a climax on Tuesday in Karbala, where thousands have already begun marching to mark the traditional 40 days' mourning that follows the anniversary of Hussein's death.

In a pilgrimage that until weeks ago brought a year in prison, they have come from Baghdad and Najaf, Nasiriyah and Basra. Some walked barefoot, in a symbol of the suffering Hussein endured. Others walked in groups, waving black, green and red flags that speak to their faith. They have crowded the roads, passing the craters and wreckage of war.

As they entered Karbala this week, many pounded their chests, a drumbeat of mourning known as lutm.

From the Shiite leadership down, the message has been that these celebrations are religious, not political. In gestures that have won admiration from skeptical Sunnis in Baghdad, clergy have reached out to their counterparts, seeking to portray unity and deflect attention from the internecine strife that many expected would erupt after the government's fall.

"No Shiites, no Sunnis, Islamic unity, Islamic unity," read one slogan on a mosque in Baghdad.

But others worry what comes next when Shiites seek to claim their political role as the majority.

"There are no problems now, but there will be problems between Shiites and Sunnis in the future," said Abu Nouri, a 66-year-old Shiite resident of Baghdad, as he gazed at the slogan. "There will be competition for power."

In some cities in Iraq, that competition has already started.

Taking Control of Cities

Abdel-Mahdi Salami, a Shiite cleric, has risen to power in Karbala. With thick-framed glasses and a beard peppered in gray, Salami led a group of 25 clergymen who have effectively seized control of the city over the last month. They have deployed hundreds of armed men, keeping the streets remarkably quiet and stanching the looting that devastated Baghdad, Basra and other cities. They restarted the civilian administration, after expelling hundreds they deemed too tainted by the Baath Party. With banks under their control and donations they receive, they have paid salaries to Karbala city workers.

Their goal, he said, is simple. "We want to make Karbala an example."

It is a trait of Shiite clergy, from Lebanon to Iran, to disavow ambition; a lust for power is considered unseemly. Salami, who likes to call himself Karbalai, or "from Karbala," has adopted that modesty. When asked who led the city's new government, run out of a dilapidated hotel for pilgrims, he looked at the ground. "Many of us," he said softly.

"You're being modest," his followers cried.

"We work sincerely, and we have no ambition for power," Salami answered. "But what we want is an administration for Karbala and the other provinces of Iraq that represents the people and delivers them what they want."

Across southern Iraq, Shiite clerics have filled the vacuum left by Hussein's fall. Although they pay allegiance to the pre-eminent seminary in Najaf, led by Grand Ayatollah Ali Sistani, they speak with their own voices, some more bold in their independence than others. Like Sadr, they claim the birthright of their own families. The father and grandfather of Salami, for instance, were prominent clergy in their own right. Others tout themselves as favorite sons, carving out territory in neighborhoods of Baghdad, or entire cities.

In Kut, along the Tigris River, Sayed Abbas, a 52-year-old Shiite cleric, has seized the city hall, surrounded by legions of armed supporters who have promised to block the entrance of U.S. forces that have yet to recognize his right to rule the city. In poor Shiite swaths of Baghdad, Mohammed Fartousi, a cleric dispatched by the seminary in Najaf, has claimed authority over an unruly collection of Shiite preachers, some of whom have installed groups of armed men at their mosques. In Najaf, the seminary under Sistani and the followers of Sadr have each moved to administer a city whose very name speaks to centuries of Shiite religious authority.

Outside their offices, powerful tribes and local leaders have flocked to pledge their allegiance -- and seek their support.

"Most of the people listen to the clergy," said Hassan Mushin Hassan, a 53-year-old cement trader in Najaf. "They have great influence over the people, and they will support the clergy to be the government. Why not?"

Suspicion of U.S. Intentions

Among Shiites like Hassan, the symbol of religious authority in Iraq is the Hawza, a seminary established more than 1,300 years ago in Najaf and long the pre-eminent center of religious learning. Until recently, clergy fluent in Persian and Arabic -- Ayatollah Ruhollah Khomeini among them -- studied theology, law and logic there with its greatest living thinkers.

It then became a backwater, repressed by the secularist Baath Party and overshadowed by the prominence given to the seminary in the Iranian city of Qom after the revolution Khomeini led. But now, just weeks after the government's fall, the Hawza has again begun to flex its muscle, inspiring hope among some that its diversity of views will bring a new vibrancy to Shiite thought long dominated by Iran. Housing 3,000 students, the Hawza sprawls around the shrine of Ali here, a collection of low-slung, brown-brick buildings.

During the war, Sistani, a cleric of Iranian origin regarded as leader of the Hawza, urged Shiites to neither support nor oppose the U.S. invasion. In the chaos that ensued, he delivered religious judgments forbidding looting. Now, with U.S. forces on the outskirts of Najaf and Karbala, the seminary has begun saying that it does not view the American presence as welcome.

"The presence of foreigners in the country is rejected," said his son, Mohammed Ridda Sistani, who acts as his spokesman.

It is a refrain echoed by cleric after cleric. Many have boycotted any dealings with U.S. officials. Some of the clerics are still bitter over what they view as the failure of U.S. forces to support a Shiite uprising after the 1991 Gulf War and blame the U.S. troops for the recent looting and lawlessness. Many remain suspicious of U.S. intentions and, in more private moments, suspect it will hand-pick a government that will deprive them of power.

But there is sharp disagreement about the tactics to organize their opposition, a dispute at the heart of the growing differences among rival wings of Shiite leadership.

In a debate that has raged for decades, the clerics in Najaf say the seminary is deeply split between a traditional wing and an activist wing. They are divided over whether clerics have a role solely in the spiritual sphere or in secular affairs. The controversy generates such great sensitivity that senior leaders refuse to even discuss it. By all accounts, Sistani subscribes to the traditional wing that shuns politics, viewing it as beneath his religious calling.

"My father does not request authority and he's not concerned with politics in any form," said Mohammed Ridda, sitting in an office near the shrine of Ali with two simple wooden tables and thin mattresses laid across the floor.

Others, like Sadr, see the clergy as a force for change, their calling intertwined with politics. As Sadr put it, "One hand with the Hawza, one hand with the people." He is sharp in his criticism of Sistani's reserve.

"From Saddam until now, he has not intervened in anything," said Sadr, who requested and was refused a meeting with Sistani.

Rivalries May Grow

The split between Sadr and Sistani is only one among a slew of rivalries and allegiances that many expect to grow amid the competition for power and influence. By tradition, all Shiites look to a senior cleric -- marja al-taqlid -- endowed with the ability to arrive at original decisions on theology and law. Sistani is among those. But he is not the only one.

Sadr, dismissed by some for his age and lack of religious learning, looks to Kadhim Husseini Haeri, an exiled religious leader considered a successor to his father.

Some pledge loyalty to Iran's supreme leader, Ayatollah Ali Khamenei, although some also question his scholarly credentials. Others, like factions of the shadowy Dawa Party, look to Lebanon's Mohammed Hussein Fadlallah, once a spiritual guide of Hezbollah.

Those rivalries were blamed in part for the killing of two clerics at the shrine in Najaf on April 10. Since then, Sistani and Sadr have gone into hiding, fearful of more violence, and some have suggested Sadr was behind a move to exile Sistani.

Sadr, sitting under a clock bearing a portrait of his assassinated father, makes clear his seclusion is temporary. He insisted he would follow the orders of his marja. "But if he gave me a choice to have a political role, I would accept it," he said, surrounded by a coterie of young advisers. "I'm ready if I receive permission."

In recent weeks, his leaflets, speaking in the name of his father and competing with Sistani's own, have gone up in Karbala. Next to them are statements from Haeri, urging Shiites to follow the teaching of Sadr's family. His followers have taken to calling the Baghdad slum of Saddam City after his father. Saddam Hospital in Najaf, where his father's body was taken, has been renamed Sadr Hospital. Across a portrait of Saddam in the capital reads the slogan, "The blood of Sadr will not go in vain."

In the interview, his son delivered his view of ties with the United States.

"I advise the Americans to ally with the Shiites, not to oppose them," he said.

He recalled what Shiites view as centuries of oppression and suffering. Added to that, he said, was the national character of Iraq -- of rebellion and dissent. "You can read history," he said The Shiites "will reject any government brought by America, any leader, any state. They have rebellion in their hearts."

A photo gallery by Post staff photographer Michael Robinson-Chavez of Shiite observances can be viewed at www.washingtonpost.com.

Shiite Muslims enthusiastically gather for prayer on streets in Baghdad's Khadimiya neighborhood, home to a revered Shiite shrine. Shiite men look out over the Euphrates River valley near the holy city of Najaf. The Shiite community's new freedom may reestablish the holy cities of Najaf and Karbala as centers of religion and politics.Young Shiite Muslims practice lutm, a Shiite ritual of chest-pounding and prayer which under Saddam Hussein was strictly forbidden. Shiite women gather for afternoon prayers at the holy shrine of Kadhimiya.A Shiite man is absorbed in his afternoon prayers at a mosque in what used to be called Saddam City and is being renamed Sadr City.Hibba, an 11-year-old Shiite in Baghdad, is among the Iraqis who have greater freedom after the defeat of Saddam Hussein. Shiites, who make up about 60 percent of Iraq's population, were repressed under Hussein.