[Update: Iraq’s Prime Minister Adel Abdul Mahdi said he would resign amid protests.]

Some armed tribal leaders joined the protesters on Thursday. The tribes represent a formidable force but so far have mostly refrained from entering the fray. But if they did rally in force to the side of the protesters, the government could have trouble maintaining control.

The governor of Dhi Qar Province, of which Nasiriya is the capital, asked Prime Minister Adil Abdul Mahdi to withdraw the Interior Ministry forces and their commander, Lt. Gen. Jamil al-Shammari. He was the one who ordered the troops to remove protesters from the bridges over the Euphrates River that they had blocked, and he had overseen the shooting.

“The ongoing campaign, with its bloody incidents since dawn, has obstructed efforts to calm down the situation, especially after our joint decision with the police command to withdraw the anti-riot forces outside the limits of Nasiriya city and not to clash with the demonstrators,” the governor, Adil al-Dikhili, wrote to Mr. Mehdi.

The governor was referring to his strategy during the past two weeks to try to accommodate the protesters who had set up tents blocking two of the bridges over the Euphrates River and to avoid clashes by having the Quick Reaction Forces withdraw rather than stay in the center city.

On Thursday at 3 a.m. the reinforcements for the Quick Reaction Forces from Baghdad, and their new commander, came with bulldozers to knock down the tents and force out the protesters. When the protesters resisted, the forces opened fire, killing at least 25 people and leaving many bleeding and injured on the ground.

The Quick Reaction Forces have previously drawn criticism for firing live ammunition and shooting tear gas canisters directly at Baghdad protesters, often giving them lethal head injuries.

By late Thursday, the military commander in charge of the forces in Nasiriya , who had been sent there just a few days earlier to rein in the protesters, had been recalled to Baghdad. The Dhi Qar provincial governor, Mr. al-Dikhili, had resigned.