U.S. Marines seized control of Tikrit today, wrapping up the last significant Baath Party stronghold in Iraq and marking a shift in the U.S. campaign from grabbing territory to targeting pockets of resistance, rebuilding war-ravaged infrastructure and creating a new system of government.

U.S. officials said the Marines who captured Tikrit, a small farming community 90 miles northwest of Baghdad and the ancestral region of former president Saddam Hussein, encountered lighter resistance than they expected, most of it coming from paramilitary Baath loyalists instead of the organized military defenses some had feared. By the end of the day, U.S. troops patrolled the city center and set up checkpoints, receiving a calm, if sometimes reserved, welcome from the population.

Marines here in the capital also refocused their duties from fighting to peacekeeping, sending out more troops to apprehend looters and starting joint patrols with Iraqi police officers. The tide of lawlessness that had enveloped the city for the last five days appeared to ebb.

Maj. Gen. Stanley McChrystal, vice director of operations for the Joint Chiefs of Staff, said in Washington that the fall of Tikrit meant "major combat operations are over" in the 26-day military campaign to take out Hussein's government. But the challenge of restoring civil order and searching for pro-Hussein militiamen remained formidable in this still-chaotic country of 24 million people.

To help with those tasks, thousands of soldiers from the Army's 4th Infantry Division moved into southern Iraq from their staging areas in Kuwait. At the same time, Pentagon officials announced that two aircraft carriers, the USS Kitty Hawk and the USS Constellation -- each with about 80 warplanes -- would depart the Persian Gulf and return to their home ports.

The Marine advance on Tikrit, one of the most heavily fortified parts of the country, was aided by several nights of fierce airstrikes on the miles-deep defensive positions that ring the town and the nearby village of Auja, where Hussein was born. As the Marines moved into central Tikrit early this morning in tanks and armored personnel carriers and under the cover of AH-1 Cobra attack helicopters and F/A-18 Hornet warplanes, the troops and militiamen assigned to guard the town melted away, as they did in Baghdad last week.

A vast presidential palace in the town's center was seized without a fight, U.S. military officials said. Although the Marines did encounter pockets of hard-core pro-Hussein fighters, Brig. Gen. Vincent K. Brooks, a spokesman for the U.S. Central Command in Doha, Qatar, said "there was less resistance than we anticipated."

U.S. commanders had expected there to be about 2,500 fighters in the town, many from the elite Republican Guard and from a paramilitary group called Saddam's Fedayeen. U.S. officials also suspected some of Hussein's top aides -- he surrounded himself with people from the Tikrit area in the belief they would be more loyal to him -- might be holed up in the town along with foreign volunteers who have attempted suicide attacks against U.S. troops.

Tikrit used to be a rudimentary farming and trading town until Hussein, who became president in 1979 but had effectively been Iraq's behind-the-scenes leader since a 1968 coup that brought the Baath Party to power, began diverting the country's oil wealth to his home town. A university was built, as were schools, mosques, hospitals and luxury residences for himself and other senior leaders. Because he distrusted people outside his extended family and those without connections to his Bu-Nasir tribe, he recruited many Tikritis to serve in top government positions and in the Republican Guard.

Small groups of Marines made several forays in and out of the town Sunday. The incursions drew occasional small-arms fire and rocket-propelled grenades but allowed the Marines to pinpoint defensive positions. This morning, they attacked Tikrit from the south, west and north, capturing a Tigris River bridge in the center, Brooks said.

After they entered the town, they began hunting for erstwhile government leaders; none was reported to have been captured. But the Marines found military equipment everywhere, abandoned by troops who had been expected to resist.

Brig. Gen. John Kelly of the 1st Marine Division, commander of the Tikrit operation, told the Associated Press that residents pointed out members of Hussein's Baath Party, Fedayeen operatives and weapons caches. In a find that officials said could provide significant intelligence, the troops discovered microfilm cartridges and hundreds of paper files in two residential buildings apparently belonging to the local Baath Party. The Marines also found a library with tens of thousands of photos of Hussein dating back decades.

Televised images of Tikrit and reports from military officials depicted the town as quiet, with few people on the streets and very little looting. Statues and portraits of Hussein -- common all over Iraq but particularly so in Tikrit -- were not vandalized as they were in Baghdad and other cities.

Here in Baghdad, more than 2,000 policemen reported back for work after requests from U.S. officials. The officers showed up at the city's Police College, where they were urged by Maj. Gen. Zuhair Nuaimi, the city police chief, to work with Marines in patrolling streets, manning checkpoints and directing traffic. By the afternoon, a few policemen clad in their green uniforms were standing alongside Marines at checkpoints downtown.

A Marine spokesman said several hundred policemen would join Marines as they patrol the eastern side of Baghdad on Tuesday. The policemen, who are not permitted to carry weapons, have been told to ride in their own cars and follow Marine Humvees as they cruise the city's streets.

One problem, however, appears to be a lack of police cars. Several were looted and many others were taken home by officers fleeing as U.S. tanks entered the capital last week. Nuaimi appealed to policemen to bring back their cars, specifically 150 sedans that had only recently been put into service.

The looting that devastated this city during the first few days after Hussein's government fell appeared to ease further as many bare ministry buildings offered no more loot and Marine patrols increased in frequency. But scattered gunfire still echoed across the city and several ransacked buildings, including the National Library with its centuries of archives, still smoldered.

Several hundred Iraqis held a demonstration in a traffic circle in front of the Palestine Hotel, where many foreign journalists are staying. The protesters complained about the lack of security and disruption of public services in the city, demanding that U.S. troops address their concerns -- but also urging them to depart quickly.

"Islamic state! Islamic state! Not American, not American!" several chanted.

Electricity was restored in parts of Kirkuk, the ethnically diverse and politically sensitive northern oil center, while U.S. troops patrolled to ensure that civil order continued to take root, four days after the city fell without a fight but descended into two days of wholesale looting.

"The number-one priority is peace and calm in the city; number two is basic services," said Brig. Gen. James Parker, who commands U.S. forces in northern Iraq.

With a couple thousand paratroops from the 173rd Airborne Brigade, a smaller number of Special Forces troops and a handful of M1 Abrams tanks and M2 Bradley Fighting Vehicles, Parker said he lacks the troop strength to do everything residents expect of a U.S. force.

Ammunition dumps remain unguarded, for instance, and one near the center of Kirkuk exploded late Sunday night, sending families fleeing through choking smoke. Also neglected has been the countryside, where ethnic Kurds have driven thousands of Arabs from their homes.

Brooks told reporters in Doha that all the oil fields in Iraq now fall within areas controlled by U.S.-led forces, but it will likely be at least a few weeks before oil is flowing from Iraq again because fields in the north and south need to be cleared of explosives and repaired.

Correspondent Karl Vick in Kirkuk contributed to this report.

A U.S. Marine tells a detained man to be quiet after catching him with looted cash and guns in Baghdad, where lawlessness appeared to ebb. A Marine holds a flag in front of a palace in Tikrit, Saddam Hussein's home town, where troops met less resistance than expected. Two U.S. marines stand in front of the palace of former president Saddam Hussein in Tikrit, which had been the last stronghold of his Baath Party.