Armed ethnic Albanian guerrillas swept triumphantly into this southern city today, fanning out into streets that were occupied by Serbian paramilitary forces just hours before and reminding NATO peacekeepers that the rebels are bent on asserting a role in a new Kosovo.

The arrival of about 300 uniformed Kosovo Liberation Army insurgents in Kosovo's third-largest city was welcomed by crowds of ethnic Albanian civilians who emerged from their houses to greet them, placing flowers in their gun barrels and showering them with kisses. Carloads of teenagers raced through the city, honking car horns and waving KLA flags. Celebratory gunfire rang out through the evening.

"Now Kosovo is ours!" shrieked Leonora Beshi, a giddy, ruby-cheeked 14-year-old who stood next to the guerrillas on the sidewalk and jumped up and down with excitement.

The rebel show of force here and elsewhere in Kosovo came as NATO forces strengthened their control over the Serbian province despite scattered incidents of violence. Humanitarian supplies began to reach internally displaced ethnic Albanians who have spent much of the last 2 1/2 months wandering around Kosovo in search of a refuge from marauding Serbian police and Serb-led Yugoslav army troops.

In Pristina, the Kosovo capital, British Lt. Gen. Mike Jackson, commander of the NATO peacekeeping contingent, said the withdrawal of Belgrade government forces from the province and the arrival of NATO troops was proceeding "pretty much on schedule," although he described the overall situation as "volatile."

U.S. officials acknowledged that Yugoslav security forces might fail to meet NATO's deadline to be out of southern Kosovo by midnight Tuesday, the first stage of a planned three-stage withdrawal. But Pentagon spokesman Kenneth Bacon indicated that allied commanders would show some flexibility in determining whether Belgrade is in compliance with last week's withdrawal agreement, noting the exodus has been slowed by clogged roads, vehicle breakdowns and a shortage of fuel and transports.

"They are certainly making a determined effort to get out," Bacon told reporters. "They seem to be doing their best to comply." Bacon said that nearly 15,000 Yugoslav troops and Serbian police have left Kosovo, taking with them roughly 20 percent of their tanks, up to 50 percent of their armored personnel carriers and more than 10 percent of their artillery.

Jackson said NATO had deployed 14,000 troops in Kosovo over the past 48 hours -- the vanguard of a force of 50,000 -- including the first detachment of 900 U.S. Marines and 150 U.S. Army troops.

After initially planning to base his headquarters at Pristina airport, only to find it occupied by about 200 Russian troops who moved in before the arrival of NATO peacekeepers on Saturday, Jackson established a command post today at an abandoned factory on the western outskirts of the city. Putting the best face possible on a major diplomatic embarrassment for NATO, he said he was "bored" by questions about control of the airfield and "very happy" to leave it in Russian hands.

Negotiations were continuing with the Russians, both in Pristina and elsewhere, to resolve the impasse at the airport and establish a clear role for the Russian military within the peacekeeping force. President Clinton and Russian President Boris Yeltsin discussed the standoff by telephone for the second straight day, and Secretary of State Madeleine K. Albright and Defense Secretary William S. Cohen plan to fly to Helsinki, Finland, this week for talks on the matter with their Russian counterparts.

Even though Russian control of the airport appears to have had a minimal effect on the pace of NATO deployment so far, a prolonged inability to use the field could complicate the logistics of the peacekeeping operation, making it more difficult to maintain and refuel helicopters and bring in supplies by air.

The United Nations, meanwhile, dispatched a convoy of relief supplies to the town of Glogovac, which lies west of Pristina in a region where tens of thousands of ethnic Albanian refugees reportedly have been hiding in the mountains after being driven from their homes by government security forces. U.N. refugee agency spokeswoman Paula Ghedini said that the four-truck convoy -- laden with baby food, blankets and hygiene supplies -- was mobbed by several thousand people when it arrived in the center of Glogovac.

U.N. officials estimate that at least a half-million Kosovo Albanians are internally displaced, meaning that they are still in the province but have been forced to seek shelter outside their homes. At least 800,000 more ethnic Albanians fled Kosovo, and the vast majority of these are still in refugee camps in neighboring Albania and Macedonia.

As NATO troops poured into the province and more and more government forces withdrew, increasing numbers of ethnic Albanian guerrillas also emerged from hiding, positioning themselves to become an important force in NATO-administered Kosovo, a province of Serbia, Yugoslavia's dominant republic.

An Albanian flag -- a black double-headed eagle on a red field -- flew today over the newly opened political headquarters of the Kosovo Liberation Army in Pristina, an abandoned school in the almost exclusively ethnic Albanian district of Kodra e Trimare. KLA members wearing red arm bands stood at attention at the entrance of the building, receiving gifts of red roses from local residents.

"Our people are all over the city," boasted a man in a sweat suit with tinted glasses and week-old facial stubble who introduced himself as Commander Cali, leader of KLA forces in Pristina.

The guerrillas, carrying automatic weapons and sophisticated sniper rifles, arrived in Prizren around 4 a.m., and by dawn they had occupied the city's northeastern quarter. "For the [Kosovo Liberation Army], this is a date never to be forgotten," said Beqir Ahmetaj, vice commander of the KLA's 2nd Battalion, who noted that the rebels, many of whom had spent months in the rugged mountains along the Kosovo-Albania border, had never before appeared openly in the city. "Today was the first day since New Year's Eve that we stepped on concrete," he said. "Some of the soldiers had difficulty walking on it. . . . It is a very happy day."

German NATO troops, who are responsible for the administration of south-central Kosovo, occasionally passed through a checkpoint set up by the KLA without stopping, good-naturedly exchanging high fives and salutes with the young rebels.

Ahmetaj, smiling at the Germans, said the KLA would play whatever role NATO desires and would disarm, if necessary. "We wouldn't like it but [will do it] if they say so," he said.

Other rebels were much less accommodating. "Too many of our people suffered, and they hungered for freedom," said Refki Destani, 28, who was guarding a pile of weapons he said were taken from the home of a local Serb. "We were the main force that brought freedom to Kosovo, and we will bring the final freedom with its independence."

That is exactly the language that most troubles Western diplomats, who have long opposed independence for the province and fear that the KLA will only reluctantly, if at all, give up its weapons and allow NATO to be Kosovo's only armed force. The agreement governing pacification of Kosovo, which was worked out by the Group of Seven industrial democracies and Russia and accepted by the Serb-controlled Yugoslav government, calls for "demilitarization" of the KLA; the guerrillas were not a party to the agreement, however, and the exact meaning of demilitarization remains unclear.

Gen. Jackson, the NATO commander, told reporters that he intends "to hold" the KLA to a commitment to demilitarize made at Kosovo peace talks in France earlier this year. "I look to them to behave with responsibility and restraint," Jackson said.

NATO commanders in Kosovo have accused the KLA of harassing retreating Yugoslav and Serbian security forces in violation of rebel promises to refrain from doing so. In the latest such incident today, a Serbian policeman was shot and severely wounded by a young Kosovo Albanian on the outskirts of Pristina as he was driving out of town.

NATO officers said they suspect the KLA was behind the shooting. Cali, the local KLA commander, dismissed the incident as an act of revenge by a young man whose parents were shot and killed by Serbian militiamen.

In Prizren today, KLA members went to the city hospital and attempted to arrest three injured Yugoslav soldiers who were being treated there, according to Lt. Col. Dietmar Jeserich, a spokesman for German NATO forces here. He said the incident was settled when German soldiers at the hospital disarmed the rebels, who then withdrew.

This afternoon, shots were fired as a 300-vehicle convoy of 5,000 Serbs, including many police officers and their families, attempted to leave town under German army escort. No one was injured, and German military officials said it was unclear who had fired the shots.

Jeserich said German troops would not allow the KLA to maintain the checkpoints it had set up and would move to disband them. The KLA also has taken over the Yugoslav border station opposite Morina, Albania, on the Kosovo frontier.

In Serbia proper, Yugoslav President Slobodan Milosevic appeared in public for the first time since the NATO air campaign began on March 24, traveling to the Danube River city of Novi Sad to speak at the site of a bridge destroyed by NATO bombs. "Now that we have peace again, new tasks await us," Milosevic told a cheering crowd. "These are to rebuild the country."

Milosevic's campaign-style appearance came as the ultranationalist Serbian Radical Party, led by Vojislav Seselj, carried out a threat to leave the Socialist-led coalition government if NATO troops were permitted to enter Kosovo. The departure of the Radicals, who occupy 82 seats in the 250-seat Serbian parliament, deprives Milosevic's Socialist party of a majority in that body. But it seems unlikely to affect the political balance of power, as the Radicals have said they have no plans to try to bring down the government or instigate public opposition to Milosevic's rule.

Milosevic associates say he is confident he can continue to govern. Analysts say the Radicals are likely to abstain on key parliamentary votes because they do not want Milosevic to strike a new coalition deal with their arch-rivals, the Serbian Renewal Movement and its leader, Vuk Draskovic, who favors reconciliation with the United States and other Western countries.

Correspondent Michael Dobbs in Pristina and staff writer Bradley Graham in Washington contributed to this report.

CAPTION: Yugoslav Withdrawal Timetable (This graphic was not available)

CAPTION: An ethnic Albanian woman pushes children in a wheelbarrow as people displaced by war head back to their homes.

CAPTION: As NATO troops assume control of Kosovo and Serb-led forces pull out, thousands of displaced ethnic Albanians returned to their homes for the first time in weeks. At left, scores descend from hiding in the hills near the central town of Glogovac, while, at right, two children get a ride back home in a wheelbarrow.