An enthusiastic welcome for U.S. forces in Najaf turned jubilant today, as several thousand Iraqis braved sporadic firefights for what one Special Forces officer described as "the Macy's Day parade," applauding a U.S. patrol that pushed close to a religious shrine at the center of the city.

Four days after encircling Najaf, the 101st Airborne Division tightened the occupation today. Three infantry battalions rolled through the streets, including neighborhoods around the venerated tomb of Ali, son-in-law of the prophet Muhammad. A company of 14 M1 Abrams tanks clanked up and down the southern boulevards, and another brigade of several thousand troops cinched the cordon on the north, seizing arms caches and swapping fire with elusive gunmen who are now believed to number no more than a few score.

In the midst of the fighting, a U.S. patrol approached Ali's tomb attempting to contact local clerics but were met instead by a crowd. Lt. Col. Chris Hughes, a battalion commander in the 1st Brigade, said, "We waited about an hour and a half, and the hair on the back of my neck began to stand up. The crowd got bigger and bigger, so we pulled back out. But it was like the liberation of Paris."

Hughes, 42, from Red Oak, Iowa, described most of Najaf as "very, very docile."

The ambiguity of occupation was fully evident today in the city, a vital Euphrates River crossroads that straddles the Army's supply line north toward Baghdad. An important Shiite Muslim cultural center, Najaf is seen by Army planners as a potential model for the subjugation and then liberation of cities farther north, particularly Hilla and Baghdad. But they acknowledged that the arrival of humanitarian aid -- some citizens here said they had been unable to find food, water or fuel for several days -- was vital to preserve goodwill.

"The desired end state is that Highway 9 is open, the airfield is open and humanitarian aid is flowing in," said Maj. Gen. David H. Petraeus, the division's commander. Even if the sporadic firefights subside, the Army expects to leave at least a brigade to secure the city, an expensive proposition for a U.S. force that is thinly spread over nearly 300 miles from the Kuwaiti border toward Baghdad. Snipers and suicide bombers remain a constant worry.

Not all of the city was peaceful today. Army OH-58 Kiowa gunships and AH-64 Apache attack helicopters wheeled overhead two by two late this afternoon, guns chattering. An Iraqi mortar round detonated with a muffled pop 150 yards from Petraeus, who stood by his Humvee, ear cocked to the blaring radios.

Down the street, a burst of gunfire slightly wounded four American soldiers. U.S. Army artillery barked in answer. An Iraqi pushing a cart with a battered suitcase walked south from the city center on shattered Highway 9. His wife and two young children held hands next to him. He sobbed great, heaving sobs, and all the agony of war was etched in his face.

U.S. reconnaissance patrols have penetrated every section of the city, poking into buildings and soliciting collaboration. "We've divided the paramilitaries into the weak, the stupid and the brave," Hughes said. "And what we've got to do is find the brave. We need the civilians to rat them out."

To further eliminate suspected strongholds, the Air Force dropped a 2,000-pound bomb this morning, demolished a building described as a Baath Party headquarters, according to a 1st Brigade staff officer.

Commanders continued to voice astonishment at the size and proliferation of arms caches around Najaf. The 2nd Brigade, near the Euphrates, found what was described as a "mine production facility" in the Kufah Technical Institute, along with more than 1,300 mines.

In the Najaf Agricultural Institute, beneath a row of gum trees on the southern edge of the city, soldiers laid out weapons discovered in tidy row houses that appeared to be owned by institute researchers or agronomists, but had recently been converted into an arsenal that included mortar rounds, grenades, illumination shells and ammunition ranging from large-caliber antiaircraft rounds to shotgun shells.

"They were in the school and in the houses and on buses. We found an AK-47 behind the bed headboards of every house," said Sgt. Jeffrey Smith, 39, of Worthington, W. Va. An Army D-9 bulldozer was dispatched to crush the weapons beneath its tracks, and then dig a pit in which munitions were to be detonated with plastic explosives.

Soldiers now occupying the institute described the cat-and-mouse games being played by Fedayeen militia forces hiding in the housing warren just across Highway 9. For the past two days, at what Smith called "the witching hour, just before sunset," an Iraqi civilian emerged on the street and sprinted toward the Americans in the institute compound.

When Army snipers opened up, killing the charging civilian, Fedayeen gunners fired at the muzzle flashes. Smith said his men replied with a barrage of confiscated rocket-propelled grenades. U.S. commanders are trying to dominate the city not only with a strong military presence but also with psychological tactics. A large equestrian statue of President Saddam Hussein, sword hoisted overhead, is targeted for ostentatious destruction by engineers with plastic explosives, said Col. Ben Hodges, commander of the 1st Brigade.

Leaflet drops also were scheduled over Najaf, with small fliers warning in Arabic, "For your safety, stay away from military forces and targets." The flip side shows drawings of hand-holding children and the Ali tomb: "We are only here to destroy military targets, not the Iraqi people."

A former Iraqi military officer from this area, who now works for the Army, noted that Najaf was a center of Shiite rebellion after the Persian Gulf War 12 years ago, but Hussein had crushed the insurrection and levied bloody reprisals.

"Most of the people here hate the regime," the former officer said. "But the people here had a hard lesson from '91. Until they see that Saddam Hussein and the regime are going to fall, they will be cautious. . . . If we make them feel like human beings, they will support us. Don't think this takes a couple hours. The regime has been in charge for 35 years."

Tactics refined here are considered preparation for what could be a much bigger fight to come. "All these things we're learning in Najaf," Hughes said, "are going to be golden when we get to Baghdad."

U.S. soldiers from the 1st Brigade of the 101st Airborne Division assault a military compound in Najaf. Commanders expressed surprise at size of arms caches found around the city.A U.S. soldier watches smoke rise from a truck filled with explosives that was destroyed by a helicopter. Four soldiers were slightly wounded in a firefight with Iraqi gunmen in the city.