After rumbling through Sadr City for much of the morning, a column of six U.S. military vehicles and a flatbed truck carrying Iraqi National Guard soldiers stopped in traffic next to an outdoor market. A child emerged from the roadside stalls, carrying a cardboard poster of Moqtada Sadr, the Shiite cleric whose strident criticism of the U.S. presence in Iraq has whipped up a large following.

On tiptoes, the child handed the poster to the Iraqi soldier manning a machine gun, as U.S. soldiers watched in dismay. The Iraqi soldier, part of a nascent security force trained and funded by the United States, held Sadr's picture aloft for a gathering, cheering mob. The convoy began moving through smoke rising from piles of burning trash on the streets of the Baghdad slum.

"If we took it from them now, this whole place would explode," said Sgt. Adam Brantley, 24, of Gulf Shores, Ala., watching from behind the wheel of a Humvee.

A week after the official handover of political authority from the United States, the Iraqi security forces are asserting, in disconcerting ways, their independence from the American soldiers who continue to serve as their protectors and patrons. Unable to shoulder Iraq's security responsibilities on their own, the Iraqi forces are nonetheless testing the limits of their new relationship with U.S. troops, including openly expressing sympathies for the most resolute enemies of the United States.

The Iraqi National Guardsmen who displayed the Sadr poster said they did so under threat of attack, and as a group they provided a useful security perimeter for the U.S. soldiers. In other regions of Iraq, more seasoned guard units have been given high marks by U.S. soldiers with the important task of training the new Iraqi security forces. On Sunday, in the city of Baqubah, 35 northeast of Baghdad, Iraqi National Guardsmen discovered a car bomb and two passengers fitted with suicide vests. In the ensuing gunfight, the car caught fire but did not explode. Both of the alleged attackers were killed.

The scene in Sadr City came a day after Sadr called on his followers to rise up against the interim Iraqi government and the foreign troops that remain in Iraq, reversing his previous stance. Sadr called the week-old interim Iraqi government "illegitimate," and promised to "continue resisting oppression and occupation to the last drop of our blood." But a Sadr aide, Mahmoud Soudani, said in Sadr City that a tenuous cease-fire with U.S. troops would remain in place, although the militia would not be disbanded until the Americans left the country.

The mixed messages left uneasy the soldiers of the 1st Cavalry Division based on an Army post at the edge of Sadr City. Earlier in the day, commanders received information from informants that Sadr's militia, known as the Mahdi Army, intended to end the cease-fire reached June 4 after two months of intense combat with U.S. forces. The fighting, which flared first in Sadr City, killed an estimated 1,500 Mahdi Army militants. The Iraqi police and national guard, then known as the Iraqi Civil Defense Corps, fled rather than fight.

Many of Sadr's surviving militants congregated in Sadr City, a desperate slum of at least 2 million people named for Moqtada's slain father, a revered ayatollah. The end of the cease-fire would likely signal a fresh round of fighting at a time when U.S. forces are determined to maintain a lower profile on the streets, hoping to give the new government and its security forces time to establish their independence in the eyes of ordinary Iraqis. But doing so is posing its own challenges as notions of sovereignty bump up against U.S. security interests.

As the crowd thickened along the narrow market street, chanting and clapping at the encouragement of the Iraqi soldiers, a shower of rocks rained on Brantley's Humvee in a staccato clatter. The convoy sped away from the market, stopping beyond the crowd's view at the edge of a dump. Sgt. 1st Class Craig Allen, 34, called down the sweating Iraqi squad leader and demanded that he hand over the poster. Finally, after an angry toe-to-toe exchange, Allen had Sadr's picture in hand.

"We wonder how these guys would react in a firefight," Brantley said, shaking his head. "Most likely drop their weapons and run away."

Since the handover, U.S. commanders with responsibility for Sadr City have required that all patrols include members of the Iraqi National Guard, a paramilitary force envisioned as an auxiliary to the Iraqi police. Only recovery missions and escort operations can be carried out without Iraqi soldiers present.

Capt. Douglas Chapman, commander of Bravo Company attached to the division's 2nd Battalion, 5th Cavalry Regiment, moved out Monday morning with four Humvees and two Bradley Fighting Vehicles. Sandwiched among them was a flatbed truck, boarded after a delay for breakfast by 15 of the newest Iraqi National Guard recruits on the post.

The convoy weaved among traffic-choked streets before parking on a wide concrete median strewn with garbage. Car repair shops lined one side of the busy road, and on the other a ditch the size of a football field opened up where a street once ran. A U.S.-funded water project began there two weeks ago, but there was not a single worker at the site.

An Iraqi interpreter, wearing sunglasses and a bandanna to conceal his identity, told Chapman that the workers had abandoned the project three days earlier. The disappearance served as a warning sign to U.S. commanders trying to fathom a murky urban war. In addition, the neighborhood children refused soldiers' offers of candy, dispensed from cargo pockets. One of them accepted a Blow Pop, only to hurl it at an Iraqi soldier.

"Ten minutes on, 10 minutes off," a shop owner lamented, referring to unreliable electricity supplies. Another man in a gray straw hat chimed in with his belief that the power is being diverted to other parts of Sadr City because of bribery. He brandished a thick stack of Iraqi dinars from his pocket to illustrate the point.

"Iraq's a sovereign nation now," Chapman told him. "This has always been up to your Ministry of Energy."

Then the children, who swarmed around the convoy, began chanting "Yes, yes, Sadr" as the troops headed toward the Humvees.

"They usually say this when we go," said Lt. Zach Swanson, 24, of Chicago. "They think it's some kind of victory."

Chapman, a 29-year-old from Great Valley, N.Y., said he was determined to resolve the electricity problem -- it was a "crucial quality of life" issue, he said, as summer temperatures push toward 120 degrees.

The convoy pulled up in front of a power station on the edge of Sadr City a half-hour later. Chapman, Swanson and the masked interpreter headed toward the entrance, where an Iraqi police officer in a pressed uniform manned the gate.

"Do you have permission to be here?" the officer asked Chapman, who if surprised did not show it. He replied yes, and a minute later the group walked toward the main building.

The three men inside the control room appeared unhappy to see Chapman, who greeted them politely before asking to see their handwritten logbook charting the amount of electricity going to each neighborhood. The antique dials and switches on the control panels gave the room the feeling of a vintage James Bond movie.

To encourage a greater sense of independence, Chapman scheduled a stop at the Habibiya police station to deliver a gift to the chief, Maj. Awad Fatlawi. It was an Iraqi flag, and Fatlawi unfolded it like a child unwrapping a Christmas present, then ordered a "Pepsi party" in his air-conditioned office while his officers jury-rigged a flag pole.

Chapman, Swanson and the interpreter, who kept the bandanna firmly on his face, sipped the cold sodas on couches lining the walls. A few floors above, several Iraqi police officers kissed the red, white and black flag before fixing it to a long stick and wiring it to the railing.

"No one gives us any weapons," Fatlawi complained to Chapman. "It is the same in every police station. We all need weapons."

Further questioning revealed that Fatlawi had a number of AK-47 assault rifles, but that there are no bullets for the 9mm pistols tucked in his officers' waistbands. Fatlawi made clear, however, that he wanted rocket-propelled grenade launchers and heavy machine guns to be able to hold off an attack. Chapman grimaced, but indicated those supplies would now come, if they come at all, through Iraq's Interior Ministry.

"Have there been any civilians helping at the checkpoints?" Chapman asked.

"No, we don't need any help," Fatlawi said, brushing away the suggestion. "Maybe they were only helping direct traffic."

Chapman made his rounds like a cop working a beat, taking the good with the bad. Then the convoy passed through the market in the center of the neighborhood, and the Iraqi soldiers gleefully waved Sadr's poster. U.S. soldiers, furious at the display, believed the demonstration incited the crowd against them.

"They've got to at least put it down," yelled Pfc. Austin Twombly, 20, of Deerfield, N.H., from the Humvee's gunner's nest as people pressed closer to the convoy. He yelled at them to do so, but they did not.

Later, defending himself in the face of several angry U.S. soldiers, the Iraqi squad leader said the child warned him to take the poster or the convoy would be attacked. "Weak leader," Allen, the sergeant, screamed at him. Chapman stepped between the men.

Back at the camp, Chapman described the challenge he faces in placing limits on Iraqi troops in a country not his own.

"They can move forward however they want," Chapman said. "We just asked them to stop displaying the picture. They can support whoever they want on their personal time."

The poster, folded in half, remained in the back of Brantley's Humvee.

U.S. troops take a poster of radical cleric Moqtada Sadr away from Iraqi National Guardsmen who were on patrol with them in Baghdad.Children from the large Baghdad slum neighborhood of Sadr City yell and throw rocks at U.S. soldiers on patrol with a unit of the Iraqi National Guard. U.S. troops argue, through an interpreter, with an Iraqi guardsman after taking away a poster of Moqtada Sadr.