“We are with it, not against it,” he said, speaking forcefully and deliberately, with a confidence he once lacked. “The government is new, and we have to open the way for it to prove it will serve Iraq’s people.”  The crowd, rowdy and at times ecstatic, answered Mr. Sadr with chants of fealty. Some cried uncontrollably. Others, dangling from electricity pylons, thrust their fists into the air.  “Yes, yes, to His Excellency, the leader!” they shouted.  The scion of one of Iraq’s most prominent religious families, who inherited a grass-roots movement founded by his revered father in the 1990s, Mr. Sadr is perhaps the sole national figure who can compete with the prominence of Prime Minister Nuri Kamal al-Maliki. So far, their relationship has proved tumultuous, from allies to enemies to allies again, and Mr. Sadr’s speech outlined the pivots on which their relationship might turn.  He insisted that no American troops could remain in Iraq after the end of this year, a condition the United States has agreed to, and urged his followers to continue to resist the troops’ presence by any means.  More cautiously, he suggested that he could withdraw support for Mr. Maliki if the government failed to address the most basic complaints of daily life here, particularly for the disenfranchised he claims to represent — shoddy roads, dirty water, leaking sewage and that persistent motif of post-Saddam Hussein Iraq, electrical blackouts.  But Mr. Sadr urged patience from his followers, and at the very least, his words seemed to mean that Mr. Maliki’s new government would have a grace period to act.  “We haven’t put a deadline on the government,” Hazem al-Araji, a prominent lieutenant of Mr. Sadr’s, said later. “We’re watching how it progresses, and after that we’ll decide.”  Another Sadr aide, Salah al-Obeidi, added, “The government needs a chance.”  The occasion itself was street theater, with an audience stretching down the street to the turquoise-domed mosque of Kamil bin Zayid. On stage was the essence of a movement many believe could transform Iraqi politics, blending the centuries-old symbolism of Shiite sacrifice and martyrdom with a martial culture fostered by the American invasion and occupation.  Since reaching its nadir in 2008, after Mr. Maliki sent the Iraqi Army against its militia in Baghdad and southernmost Basra, with decisive help from the American military, the movement has returned to prominence, essentially by staking its claim to Iraqi politics. It made a strong showing in local elections in 2009. The next year, Mr. Sadr, who had long hedged his support for elections, called on his supporters to fully participate in the vote last March. They did, and in a feat of logistics and planning, the movement won 40 seats, emerging as a savvy player in the eight months of negotiations that followed. Its eventual support for Mr. Maliki was the key to his return for a second term.  At the speech on Saturday, the movement sought to convey a certain respectability, from sharply dressed guards in gray suits to the punctuality of Mr. Sadr himself, who began speaking precisely at 10 a.m., as scheduled. Cadres passed out Iraqi flags, which soon overwhelmed the crowd’s banners of pious slogans and portraits of Mr. Sadr.  In fact, it was hard not to draw at least superficial comparisons to another Arab Shiite movement, Lebanon’s Hezbollah; both navigate political, social and military identities and have built personality cults around their leaders, junior clerics who rose to prominence as politicians.  There were even echoes of Hezbollah’s leader, Hassan Nasrallah, though Mr. Nasrallah remains far more dynamic and eloquent than Mr. Sadr. It was there in the mannerisms, the way each held a white handkerchief or excused himself to drink a glass of water. Each related to his audience the same way, too — speaking in a stentorian formal Arabic, only to lapse into slang when offering a joke or a casual aside.  “What’s up? Are you scared of the Americans?” Mr. Sadr asked from a 25-foot-high pulpit, draped in black, before leading the crowd in a chant of “No, no to America.”  The crowd answered defiantly, raising their voices.  “You’re getting better,” he complimented them, with the wriest of smiles.  Most comparisons to Hezbollah go only so far, though, and many Sadrist officials point out the obvious: Just as Hezbollah was impossible without Israel’s 1982 invasion of Lebanon, so is Mr. Sadr’s movement without the assassination of his father on Mr. Hussein’s orders in 1999 and the American invasion and occupation four years later.  “We are a movement, we are not simply a party,” said Hakim al-Zamili, a Sadrist lawmaker who beat charges of murder, kidnapping and corruption from his time as a deputy minister in the previous Maliki government.  With that ambiguity, Mr. Zamili captured the challenges of an oppositional movement joining the mainstream without surrendering the legions of supporters from hardscrabble neighborhoods, so zealous that some tried to stampede the stage.  Mr. Sadr’s speech offered few clues as to how he would bridge that divide, going little beyond the movement’s reflexive nationalism and millenarian theology. Nor did his lieutenants, some of whom volunteered a contradictory notion of being inside the government looking out, and from there looking back in at its performance.  “We’ll be watching them,” said Nassar al-Rubaie, the minister of labor and social affairs. “We’ll monitor the performance of this government.”  Khalid D. Ali contributed reporting.  A version of this article appears in print on January 9, 2011, on Page A1 of the New York edition with the headline: Once Enemy Of Iraqi State, Cleric Joins It. Order Reprints| Today's Paper|Subscribe
To a rapturous welcome, the populist Iraqi cleric Moktada al-Sadr delivered his support for an Iraqi state that he had once derided as a tool of the United States.