Shiite cleric Moqtada al-Sadr’s militia plans an elaborate return here Thursday, when tens of thousands of supporters are expected to join it in a march to demand the withdrawal of U.S. forces from Iraq, a defiant show of political and military muscle in an already tense capital.

Sadr’s recent efforts to reinsert himself in Iraq’s political process after nearly four years of self-imposed exile in Iran are putting additional pressure on Iraqi and U.S. officials grappling with fresh security challenges and the question of whether any American troops will remain in the country after this year.

Sadr has called for his Mahdi Army to march through Baghdad’s predominantly Shiite Sadr City, a slum that was once a hotbed of anti-U.S. violence. The parade will include renewed calls for U.S. forces to abide by a Dec. 31 withdrawal deadline.

Militia members will not be armed, but Sadr loyalists have warned that could change if U.S. forces remain. Such a move by the militia could hobble Iraq’s efforts to move beyond years of bloodshed.

“The march is a message to the occupier that we are serious when we say, ‘In case the occupier will not leave and the political blocs agree on giving an extension to the occupation, we will start with peaceful strikes. Then the freezing of the al-Mahdi Army will be lifted,’ ” said Jawad al-Hasnawi, a Sadrist member of parliament, referring to Sadr’s 2008 order to the militia to disarm.

Radical Shiite cleric Muqtada al-Sadr speaks at Friday prayers in Kufa, 100 miles south of Baghdad, Iraq, Friday, May 13, 2011. (Alaa al-Marjani/AP)

During the height of the insurgency after the U.S.-led invasion in 2003, Sadr and his militia were blamed for much of the sectarian and other violence that killed thousands of American troops and nearly plunged Iraq into civil war.

With the support of then-President George W. Bush, Iraqi Prime Minister Nouri al-Maliki crushed the Mahdi Army in late 2007 and 2008, sending Sadr into hiding until earlier this year.

Sadr and his top advisers have promised that Thursday’s protest in Sadr City — scheduled to coincide with the anniversary of the birth of the prophet Muhammad’s daughter — will not trigger more violence in a country that faces almost daily bombings and assassinations.

Instead, the march is being billed as a celebration of national pride and an opportunity for Sadr’s followers to declare their intent to be part of Iraq’s future. During the rally, the Mahdi Army plans to show off new uniforms bearing the Iraqi flag to “express the unity of Iraqis,” said Haim Alaraji, a Sadr adviser.

Still, Iraqi security forces have stepped up patrols in the capital, anticipating the arrival of potentially large groups of protesters from across the country. The U.S. Embassy also urged American citizens in Iraq to be extra vigilant this week and avoid large gatherings, including the Mahdi Army’s demonstration.

The march comes at a delicate time for Maliki as he weighs whether to request an extension for some U.S. troops.

Defense Secretary Robert M. Gates and some congressional leaders have said that they would like to keep some troops in Iraq to help in the training and security of Iraqi forces. Maliki has suggested that he is open to a continued U.S. presence, but only if most of the country’s main political blocs agree.

According to a report in Iraqi media this week, Maliki plans to meet with bloc leaders in the coming days to discuss a plan that would allow up to 20,000 U.S. troops to remain, about half of the 46,000 currently in the country.

Sadr hinted after a sermon this month that he could be persuaded to agree to letting some troops remain if the extension was part of a broader political agreement between the Sadrists and Maliki, also a Shiite.

But Ali al-Kufi, Sadr’s head of media affairs in Najaf, told The Washington Post on Wednesday that a continued U.S. presence could quickly turn the Mahdi Army back into a fighting force.

“The march is a way we say . . . in case the American forces insist on staying . . . we have a military choice to face them,” Kufi said.

State Department officials declined to comment Wednesday on Sadr’s planned demonstration.

Although Sadr and his top advisers have been speaking up more in recent weeks on developments inside Iraq, many observers say they doubt that he wants to renew armed conflict in the country. The Sadrist movement has also shown signs of splintering as it transitioned in recent years from a military to a political force, with a voice in Maliki’s cabinet and in parliament.

Liqaa al-Yaseen, a Sadrist lawmaker, noted that march participants have been told that “no flags and no pictures should be raised but that of Iraqi national flags.”

“Our message is: We are working to liberate Iraq, build Iraq and improve our homeland, but not alone,” Yaseen said. “Iraq cannot be liberated and built only by one part of Iraqis. All Iraqis must cooperate.”

Majeed is a special correspondent. Special correspondents Aziz Alwan in Baghdad and Saad Sarhan in Najaf, Iraq, also contributed to this report.