Marine commanders working in unheated, war-damaged downtown buildings are hammering out the details of their paradoxical task: Bring back the 300,000 residents in

time for January elections without letting in insurgents, even though many Fallujans were among the fighters who ruled the city until the US assault drove them out in November, and many others cooperated with fighters out of conviction or fear.

One idea that has stirred debate among Marine officers would require all men to work, for pay, in military-style battalions. Depending on their skills, they would be assigned jobs in construction, waterworks, or rubble-clearing platoons.

"You have to say, 'Here are the rules,' and you are firm and fair. That radiates stability," said Lieutenant Colonel Dave Bellon, intelligence officer for the First Regimental Combat Team, the Marine regiment that took the western half of Fallujah during the US assault and expects to be based downtown for some time.

Bellon asserted that previous attempts to win trust from Iraqis suspicious of US intentions had telegraphed weakness by asking, " 'What are your needs? What are your emotional needs?' All this Oprah [stuff]," he said. "They want to figure out who the dominant tribe is and say, 'I'm with you.' We need to be the benevolent, dominant tribe.

"They're never going to like us," he added, echoing other Marine commanders who cautioned against raising hopes that Fallujans would warmly welcome troops when they return to ruined houses and rubble-strewn streets. The goal, Bellon said, is "mutual respect."

Most Fallujans have not heard about the US plans. But for some people in a city that has long opposed the occupation, any presence of the Americans, and the restrictions they bring, feels threatening.

"When the insurgents were here, we felt safe," said Ammar Ahmed, 19, a biology student at Anbar University. "At least I could move freely in the city; now I cannot."

A model cityUS commanders and Iraqi leaders have declared their intention to make Fallujah a "model city," where they can maintain the security that has eluded them elsewhere. They also want to avoid a repeat -- on a smaller scale -- of what happened after the invasion of Iraq, when a quick US victory gave way to a disorganized reconstruction program thwarted by insurgent violence and intimidation.

To accomplish those goals, they think they will have to use coercive measures allowed under martial law imposed last month by Prime Minister Iyad Allawi.

"It's the Iraqi interim government that's coming up with all these ideas," Major General Richard Natonski, who commanded the Fallujah assault and oversees its reconstruction, said of the plans for identity badges and work brigades.

But US officers in Fallujah say that the Iraqi government's involvement has been less than hoped for, and that determining how to bring the city safely back to life falls largely on their shoulders.

"I think our expectations have been too high for a nascent government to be perfectly organized" and ready for such a complex task, Colonel Mike Shupp, the regimental commander, said at his headquarters in downtown Fallujah.

While one senior Marine said he fantasized last month that Allawi would ride a bulldozer into Fallujah, the prime minister has come no closer than the US military base outside the city.

The Iraqi Interior Ministry has not delivered the 1,200 police officers it had promised, although the Defense Ministry has provided troops on schedule, US officials said. Iraqi ministry officials have visited the city, but delegations have often failed to show up. US officials say that is partly out of fear of ongoing fighting that sends tank and machine-gun fire echoing through the streets.

Meanwhile, the large-scale return of residents to a city where only Humvees and dogs travel freely will make military operations as well as reconstruction a lot harder. The military must start letting people in, one neighborhood at a time, within weeks if Fallujans are to register for national elections before the end of January. The government insists the elections will proceed as scheduled despite widespread violence.

The Marines say several hundred civilians are hunkered down in houses or at a few mosques being used as humanitarian centers. In the western half of the city, civilians have not been allowed to move about unescorted. In the eastern half, controlled by another regiment, they were allowed out a few hours a day until men waving a white flag shot and killed two Marines.

"The clock is ticking. Civilians are coming soon," Lieutenant Colonel Leonard DiFrancisci told his men one recent evening as they warmed themselves by a kerosene heater in the ramshackle building they commandeered as a headquarters. "It's going to get a lot more difficult. We've had a little honeymoon period."

A tall order If DiFrancisci's experience dealing with a small delegation of Iraqi aid workers is any indication, sorting out civilians from insurgents in large numbers will be overwhelming.

One afternoon last week, DiFrancisci, a reservist from Melbourne, Fla., and a mechanical engineer, was ordered to escort workers from the Iraqi Red Crescent Society out of the city on their way back to Baghdad. The Red Crescent, an equivalent to the Red Cross, had been butting heads for days with Marines who initially denied the aid organization entry to the city, insisting the military was taking care of civilians' needs. The society finally won a Marine escort in and refused to leave, setting up in an abandoned house.

Dr. Said Hakki, the group's president, met DiFrancisci and Lieutenant Colonel Gary Montgomery at a mosque, eager to mend fences. "We want to play by your rules," Hakki said.

Montgomery agreed that Marines would ferry a group of aid workers to Baghdad, along with several women and children who had been rescued from houses. But when the Humvees pulled up to the Red Crescent house, scores of young men who had taken refuge there were milling around the streets. There was no way to tell whether they were fighters.

"All these military-age males are out during curfew," Montgomery told Hakki. "If you all don't follow the rules, you're going to get people killed."

Tensions rose when about a dozen women and children started climbing into ambulances for the ride to Baghdad. One man tried to get in, gave the Marines who challenged him several versions of his age, then decided not to go rather than discuss it further.

Suhad Molah, a young woman in a veil that showed only her eyes, was indignant that a translator said she might be Syrian because of her accent, implying she was the wife of a foreign fighter.

"I am Iraqi," she said, adding that she and her children had been trapped in their house for weeks.

The Marines were also suspicious when more than a dozen men, not the handful they expected, said they were Red Crescent staff members headed back to Baghdad. Some had no identification, and there was no way to verify whether they were the same men who had come out from Baghdad.

"This is not a 'muj' rescue service," DiFrancisci said, using slang for mujahideen, or holy warriors. Montgomery remarked, "The real negotiations start after you've agreed on something."

The Marines let the men go after Hakki vouched for them, but not before the Iraqis grew angry that their motives had been questioned. The convoy headed onto the highway, but only after a dozen Marines had spent two hours organizing and searching the vehicles. Back at their headquarters, the team debated the procedure for allowing civilians to return. Major Wade Weems warned that there should be a set number per day so that a backlog would not form behind the retina-scanning machine, fueling resentment.

When they heard of the proposal to require men to work, some Marines were skeptical that an angry public would work effectively if coerced. Others said the plan was based on US tactics that worked in postwar Germany. DiFrancisci said he would wait for more details. "There's something to be said for a firm hand," he said.

Anne Barnard can be reached at abarnard@globe.com.