Marching under palm fronds and the black banners of their faith, Baghdad's Shiites set off by the thousands for an emotional revival of an annual religious pilgrimage discouraged for decades by Saddam Hussein.

For young Shiites like Ahmed Kasam, 22, among throngs of faithful streaming south Saturday along the gravel medians and palm groves lining Baghdad's highways, "This is my first time for the march."

For older men like Hussein Saman, 48, imprisoned for 11 years for openly practicing Shiite rituals, bare feet were taking him toward one of his sect's most sacred shrines, Karbala, for the first time since the early 1970s, the first years of Baath Party rule.

"In the days of Saddam, if anyone did this march, he was killed," said Saman, in dirty gray caftan cinched with a frayed wire. "The least penalty was prison, for life."

As many as 2 million Shiites from Iraq, Iran and other countries are expected to converge this week on the holy cities of Najaf and Karbala in Iraq. The pilgrimage represents a major test of the order U.S. forces are seeking to establish in Iraq.

At least one leading Shiite cleric -- Ayatollah Mohammed Baqir al-Hakim, head of an Iran-based anti-Saddam movement -- has called on the Karbala march to be used to demonstrate Shiites' rejection of U.S. rule.

U.S. troops are lying low to avoid stirring up anti-American feelings, even as they stockpile emergency food and water for the pilgrims and work out security measures for the gathering. There are also concerns the event could be disturbed by tensions between rival Shiite groups that have erupted into violence since Saddam's fall.

"We don't want to interfere with the pilgrimage. We want it to proceed as normally as possible. But we are prepared for the worst," said Maj. James M. Bozeman, a civil affairs officer with the U.S. Army's 82nd Airborne Division.

The worst, it seems, would be U.S. troops firing on pilgrims to restore order, giving credence to hardline clerics who say their country is being occupied by a hostile power.

The ideal, from the U.S. viewpoint, would be a trouble-free pilgrimage, with Shiites giving credit to transitional authorities.

Shiites, who make up 60 percent of Iraq's 24 million people, were repressed under Saddam Hussein's regime, which was dominated by Sunni Muslims. Now with Saddam gone, they are showing their strength.

As the United States works to put together a post-Saddam government, Shiite clerics have emerged as key centers of political power, helping organize local administrations to maintain order.

Iraq's top Shiite cleric, Grand Ayatollah Ali al-Sistani, issued a statement Friday rejecting "any foreign rule in Iraq."

Al-Sistani, however, disavowed any political ambitions, saying he "by no means whatsoever is looking to establish (himself) as a political authority in Iraq." The grand ayatollah is based in the southern holy city of Najaf. His rulings are followed by clerics and faithful throughout the country.

Open observance of the pilgrimage is the latest mark of Shiite flourishing since Saddam's fall.

Mosques in some Shiite neighborhoods now resonate each day with the slapping thuds of thousands of men at once pounding their bare chests with their fists in a ritual of contrition -- a ritual banned under Saddam. Many still practiced it secretly, in their homes.

The fallen regime permitted annual pilgrimages, but it prohibited movement on foot and monitored the participants as well as the centers of Shiite rebellion in Najaf and Karbala.

Hundreds of the faithful took to the road Saturday for Karbala, chanting prayers and carrying banners emblazoned with Quranic verses. The main pilgrimage route runs 45-mile from Najaf to Karbala.

The gathering marks 40 days, or al-Arbe'ein, after the date on which Hussein -- a grandson of the Prophet Muhammad and one of the Shiite Islam's most revered saints -- is believed to have been killed in Karbala. Ashoura, which marks the day he was killed, fell this year on March 12 under the lunar Islamic calendar.

American units like the 82nd, providing security for the pilgrims, were keeping their distance.

Thousands of humanitarian rations, as well as food and medicines -- stockpiled by the U.S. military for the pilgrims -- will be handed out by Iraqi authorities, not Americans, Bozeman said.

Religious leaders in Najaf have worked out an agreement with the American military to keep U.S. troops at least 500 yards away from the burial shrine of Hussein's father, Imam Ali, son-in-law of the Prophet Muhammad and regarded by Shiites as his successor.

Under Saddam, checkpoints set up on the perimeters of Baghdad turned back any pilgrim who defied the restrictions and tried to make the three- or four-day march to Karbala.

"Even though it is a march of mourning, today we are happy, because we are doing it freely," one man said Saturday, walking on too quickly in the crowd to give his name.

Happy as they were to see Saddam gone, none of the marching Shiites expressed any gratitude for the U.S. forces who toppled him.

"We fought Saddam, and if America is unjust to us, we'll fight her," Saman said, marching on.