As a police officer in Iraq's old system of dictatorship, Kareem Hassan was one of the untouchables, a swaggering symbol of authority in his green uniform and black beret. But with the collapse of President Saddam Hussein's government, his role has suddenly changed.

"Now the situation is reversed. I am afraid of the citizens," Hassan, 46, a short, portly man with graying hair, admitted bluntly, as sat down to a lamb kebab at the end of a patrol today.

After nearly a week of chaos and looting, Hassan and scores of other policemen have suddenly reemerged as part of an Iraqi security force being reconstituted by the U.S. forces that now control the country.

The Iraqi police are conducting joint patrols with Marines, an exercise intended to curb the disorder in such cities as Baghdad and Basra. But the collaboration represents something more profound: the first step in rebuilding Iraq's institutions.

"What you are seeing is akin to what happened in 1945 when Berlin fell. Every employee had to be a member of the party" under the Nazis, said Capt. Joe Plenzler, spokesman for the 1st Marine Division, which is helping relaunch the police force.

He suggested that many Iraqi officials could find a place in a new, more democratic Iraq despite their association with Hussein's authoritarian government and ruling Baath Party. "There are a lot of good, well-intentioned people," he said.

Whether Iraq can be transformed into a state with the rule of law is open to question. The experience of the United States and other countries shows that it is often difficult to reform traditionally corrupt police and judicial institutions. In a sign of possible trouble, a few Iraqis have jeered the returning police officers.

But many Iraqis seem to blame the police more for petty corruption than for the torture or assassinations ascribed to other Iraqi security agencies. Today, citizens appeared to give the officers the benefit of the doubt as they swept through the streets in white police cars sandwiched between Humvees topped by machine-gun-wielding Marines.

In the past, said Shant Serkis, 49, owner of an auto parts store, "the police stole from the people." But he was thrilled to see the Iraqi and American forces parked in his neighborhood, because looters had been pillaging. "We are thankful for the cooperation between the two sides," he said.

During the war, Iraqi police simply walked off the job, hiding their uniforms and cars so they would not be attacked by mobs furious with Hussein's government. In recent days, U.S. and British authorities have invited them to return to work. In Baghdad alone, more than 2,000 have done so.

Maj. Andrew Petrucci, 31, a Marine working with the Iraqi police, said U.S. civil affairs officers had screened the returnees to weed out thugs. The U.S. military is providing some weapons to the poorly equipped officers, while warning them to respect citizens.

Iraqi police officials said they were working hard to project a kinder, gentler image.

"If anybody complains we are no good, the Americans will capture us and put us in a prison," said one police officer, Almar Rasheed, 27.

The Iraqi police appeared to have mixed reactions to working with the forces that only days ago had been their enemies. Some were confused by having both Iraqi and American bosses. Some seemed nervous. Others were just awe-struck.

"The Americans are very kind. They respect any person. This is a good value," said Halid Jamil, 26, a policeman, shortly before he took off in a convoy with the Marines today.

Jamil's patrol offered a glimpse into the enthusiasm of the Iraqis. Just five minutes after Jamil hit the road, his white police car shrieked to a halt. He had spotted two men trying to drive a government truck away from the Ministry of Local Affairs. The skinny, small-boned Jamil shot out of the car and sprinted across the lawn of the ministry building, with the gear-laden Marines hoofing behind.

Jamil walked back gripping two sullen young men by the arms. "Thieves," he declared to the Americans, proudly depositing his trophies in a police car.

A few minutes passed as they waited for the rest of their patrol to return from the ministry parking lot. Jamil liked being with the Americans.

"My uncle, in Detroit," he said.

A Marine tried to communicate back. "Ali Babas," he said, pointing to the two detainees, using Iraqi slang for thieves. "Lock them up."

But before the convoy could take off, their next target appeared: a battered white van, its windshield smashed, weaving down the road. A man inside waved an open bottle of vodka. The Iraqi police nervously informed the Americans that the occupants appeared dangerous. But the Marine leader told his men it was the Iraqis' responsibility to check the vehicle.

"They do their jobs. We provide security from the crowd," explained Staff Sgt. Kevin Fountain, 32, of Twentynine Palms, Calif. "We need to make sure nobody in the population gets at 'em."

A few minutes later, Jamil emerged from the suspect vehicle with another prize: an AK-47 rifle that the occupants had hidden. "Dangerous," he gravely informed the Americans, brandishing the firearm.

While patrols such as Jamil's carry symbolic weight, the Iraqi police provide only the illusion of a criminal justice system. The Americans still control Baghdad and other cities. The Iraqi officers draw no salaries, as there is no government. There are no jails, either, because existing facilities were abandoned by their staffs and ransacked.

"That's on their priority list, some kind of detainee center," said Petrucci, the Marine helping the police force. "The next step will be getting the judiciary system up and running."

The Marines accompanying Jamil's patrol today expressed a certain amusement as they noted the Iraqi officers' crude manner of working.

"I didn't see a roadside sobriety test," one noted as Iraqi policemen locked the three occupants of the white van in the van's rear cargo area. The two young men apprehended at the ministry were then added to the van's contents. Also placed inside was a passenger of a taxi the police had stopped, who was carrying a computer that appeared to have been stolen.

"A new paddy wagon," declared a Marine watching the van fill up.

"I think this is a little different from our system," said another.

But to Jamil, the work felt important. Police work in Iraq had changed greatly in a matter of days, he said. Previously, he said, the police were a beaten-down part of the system, disliked by the public but under the thumb of the feared security services linked to Hussein.

"In the past, I could never talk to an American journalist. The security would not allow me," he said through an interpreter.

Soon, it was time to head back to the city police academy, where the new local force is based. Jamil took the wheel of the seized white van. "Come in here beside me," he said to one of his new Marine friends.

"Hell, no!" exclaimed the American, looking at the broken-down vehicle. Then, he softened his tone. "We're not allowed to," he explained.

Jamil looked dejected. "I want anybody of the Marines to sit with me," he said.

The convoy took off. Across the street a crowd of several dozen Iraqis from the neighborhood had gathered, including Serkis, the owner of the auto parts store.

"This is a new experiment for us," he said of the joint patrols. "We need a few days to see this, see how it works."

But his early assessment: The new police had passed the test.

A member of Baghdad's reconstituted police force points out a stolen car that he had stopped during a patrol with U.S. Marines. Returning officers were screened by U.S. forces.U.S. Marines cover Iraqi policeman Mokhalad Majed as he looks for looters in the Ministry of Local Affairs in Baghdad, where two men were driving a government truck away from the ministry.