Scarcely a street corner can be found without a Mahdi Army member, more often than not in a black shirt with a pistol tucked discreetly in his waistband. Sadr officials say the group is making the first tentative steps towards becoming a political force like Hezbollah in Lebanon.
US patrols rarely venture here and the local police tend to take orders from Sadr's men rather than the other way around. Every afternoon, large queues of supplicants form outside Sadr's main office to ask for help with medical bills, schooling, and jobs.
By running a wildly popular anti-vice campaign in cooperation with local police, Sadr's men - and not the US-installed interim government - have taken up the mantle of chief guarantors of public order in Sadr City. Mahdi Army members have killed alleged drug dealers and kidnappers, and handed more over to the police. Local cops confirm their cooperation. "They're doing a lot of our work for us,'' says one.
Though it has little money and doesn't support anything like the wide array of medical clinics, mosques, and social services that have cemented Hezbollah's popularity in parts of Lebanon, the Mahdi Army is the most potent social and political movement in Sadr City. The area holds about 10 percent of Iraq's electorate - a powerful bloc in a country divided between the Kurds and competing Shiite and Sunni factions.
Mahdi Army member Sheikh Saadi lives in a narrow apartment of rooms with his wife and infant son in what was once a single family home but has been divided into three. Most Sadr City dwellings have been subdivided to accommodate the booming population.
He rolls up a pant leg to show a gunshot wound he says he received while leading an operation against a kidnap-for-ransom gang about seven weeks ago. The alleged kidnappers? "They're no longer with us,'' he says with a smile.
"We know everyone here, the good people and the bad people, and we're dealing with them." He also cheerfully confirms that the Mahdi Army has been behind the recent fire-bombings of liquor stores and video stores they accuse of selling pornography. "We're protecting people from these things."