BAGHDAD, April 9 -- Draped in Iraqi flags and chanting anti-American slogans, tens of thousands of Iraqis swept into the southern city of Najaf on the call of radical Shiite cleric Moqtada al-Sadr to mark the fourth anniversary of the ouster of President Saddam Hussein, calling for U.S. forces to withdraw from Iraq.

"No, no to the occupier. Yes, yes, to Iraq," they chanted, as demonstrators burned and ripped apart American flags. "Get out, get out occupation."

Others carried banners proclaiming their loyalties to the influential cleric. They had traveled by bus and cars, from Baghdad and Basra, to march peacefully, under heavy security, through the center of one of Shiite Islam's holiest sites.

"We came today raising this flag, our flag, the flag of Iraq, as a show of unity," said Ali Hamza, 26, from Sadr City, the cleric's Baghdad stronghold. Hamza wore the trademark black uniform of the Mahdi Army, Sadr's militia, and an Iraqi flag covered his back. "I would like to show the world we are united. We reject the occupation and we will fight the occupation."

Sadr has not been seen in months, leading critics to speculate that his grip over his followers has weakened and that he has fled the country to neighboring Iran. But Monday's protest underscored the immense power Sadr still wields even as an eight-week-old U.S. and Iraqi security offensive to quell violence in Baghdad has targeted many of his commanders and advisers, as well as areas controlled by the Mahdi Army.

"Today, we have proven to the entire world in this demonstration that when Sayyid Moqtada Sadr called on them, they all came, despite all the dangers, from all the provinces," said Mahmoud al-Kufi, the security chief at Sadr's mosque in nearby Kufa, using the honorific for a person descended from the prophet Muhammad.

"Now we have three times the power and force of what he had in 2004," he added, referring to the year when U.S. troops and the Mahdi Army fought fierce battles in Najaf and elsewhere.

The protests came a day after Sadr, in a statement, called America "the great evil" and urged his militiamen and members of Iraq's security forces to unite against the U.S. occupation. But he stopped short of calling for a violent uprising, which would almost certainly complicate current efforts to stabilize Iraq.

Sadr, a nationalist who has long called for a U.S. withdrawal, is engaged in a delicate balancing act. His street power is largely derived from his opposition to the U.S. occupation, yet his political bloc is also part of Iraq's ruling coalition, which is trying to stabilize the capital. He has ordered his fighters to stand down and not be provoked into battle as U.S. troops patrol and conduct security sweeps.

On Monday, his men said they were itching to fight U.S. and Iraqi troops and that Sadr's commands were the only thing stopping them. "We wish as much as the distance between heaven and earth," said Kufi. "When he asks us to fight, we will fight and hope for martyrdom."

The demonstrators began Monday morning at the Kufa mosque, where Sadr has delivered Friday sermons, and headed toward Sadrain Square in the center of Najaf.

Some carried signs that read: "Four hard years and Iraq is from bad to worse."

The protesters were largely Shiite, but included Sunni politicians and clerics. As they walked, they held hands with black-turbaned Shiite clerics.

"We came to participate in this great patriotic national demonstration to denounce the occupation and say to them, 'Get out, so we can build our country with our own hands,' " said Ala Adin, a leading Sunni imam from Basra.

Armed Mahdi Army militiamen, in black, manned checkpoints throughout the city, along with a few Iraqi policemen. Some militiamen wore caps that read: "One Prosperous Iraq."

At Sadrain Square, Abdul Razaq al-Nadawi, a Sadr spokesman, walked onto a stage and declared to the crowd: "A few days ago the father of evil, Bush, went out and said: We are staying in Iraq until the mission is accomplished according to the authorization of the U.N. and the request of the Iraqi people. So what would you answer?"

"No, no, America. Get out, get out, occupier," protesters yelled.

Sarhan reported from Najaf. Special correspondents Saad al-Izzi, Naseer Nouri and Waleed Saffar in Baghdad contributed to this report.