BAGHDAD – Shiite cleric Muqtada al-Sadr’s militia plans an elaborate return here today, 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-American violence. The parade will include renewed calls for U.S. forces to abide by their scheduled Dec. 31 departure.

Although militia members will not be armed, Sadr loyalists have warned that could change if U.S. forces remain. Such a move by the militia could hobble efforts to move beyond 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 member of parliament, referring to a 2008 order to the militia to disarm.

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.