With the last Yugoslav troops scheduled to depart Kosovo on Sunday, NATO forces are preparing to escalate the disarming of ethnic Albanian guerrillas who are trying to take over police duties and committing violence against rival Serbian communities.

Even though NATO and Kosovo Liberation Army officials still have not signed a planned agreement setting the timetable for what is officially termed "demilitarization" of the separatist rebels, allied forces already have confiscated hundreds of weapons from KLA members. NATO also has intervened in numerous conflicts involving Serbian citizens, whom the guerrillas consider sympathetic to the Serb-led Yugoslav forces that brutalized ethnic Albanians in Kosovo.

U.S. forces, which include the Marines and the 82nd Airborne Division, have been the most aggressive of the NATO troops in stripping KLA members of their weapons, arresting recalcitrant rebels and interceding in incidents of violence and threats against Serbs.

Today U.S. Army troops escorted north five buses filled with as many as 400 Serbs who were rousted by armed KLA members from their homes in this southern Kosovo city three days ago. The KLA then herded the Serbs into the town square, where they were beating and threatening to shoot some of them as members of the 82nd Airborne Division arrived to take control of the city, said U.S. Army officials.

"Some had been beat up and tied up, they [KLA rebels] were pointing their guns at them," said one U.S. Army official who saw the incident. "They probably would have started shooting. They're mad and they're looking for revenge."

The U.S. soldiers arrested 14 guerrillas and confiscated their weapons, although the KLA rebels were released later by Army civil affairs authorities who said NATO has no authority or provisions for detaining KLA members.

The troops then searched the frightened Serbs and took away even more weapons, one Army official said, noting, "Even old women had grenades in their pocket books."

The soldiers allowed the Serbs to take refuge in the municipal building where the U.S. forces have established a headquarters and "put guards on the stairs to protect them from KLA or drive-by shootings," said the Army official.

In the wake of the intimidation in the town square, thousands of Serbs fled Urosevac leaving the town empty of virtually its entire Serbian population, Army officials said. Although the KLA became a useful ally to NATO during its 78-day air campaign against Yugoslav forces, the guerrilla army has a reputation as a violent and repressive organization.

"Now Serbs are scared," said one U.S. Army official who observed the past three days' events in this city.

The incident in the town square preceded another episode in the southwestern town of Prizren yesterday. German NATO troops there raided a former Serbian Interior Ministry police headquarters and disarmed about 25 KLA rebels who apparently had imprisoned and severely beaten 15 elderly people, including a man found dead and chained to a chair, German military officials said. Just as the Americans had done, the Germans arrested, then released the KLA members.

While NATO and KLA officials have fallen behind schedule in their efforts to reach an accord for demilitarization of the rebels, individual NATO commanders have begun filling the vacuum by setting their own rules for rebel behavior.

Americans have been searching cars passing their checkpoints and confiscating weapons from KLA members and ethnic Albanian and Serbian civilians. The Germans, who control the southwestern part of the province, have cut a separate deal that, as of midnight Friday, KLA units within a mile of Prizren's main roads will not display their weapons publicly and will be allowed to wear their uniforms for only about another month. The German military today also was seen taking guns away from KLA members driving through Prizren.

While departing Yugoslav forces have been burning and looting houses as they leave the country, ethnic Albanians also have been setting Serbs' houses afire as they return to their abandoned neighborhoods. Near the town of Klina in central Kosovo, two Serb homes were afire on a hilltop above neat rows of vegetables.

NATO officials said the last 3,000 to 5,000 Yugoslav military troops in Kosovo were scheduled to leave by noon Sunday, 12 hours before their midnight deadline under a military agreement signed by NATO and Yugoslav generals.

Several hours after the busloads of Serbs left Urosevac today, Peuraca Gojko, 60, sat with his wife and four children next to sacks of their household possessions. An ethnic Serb who was forced to flee Croatia five years ago, he now found himself a refugee once again.

"I have so many Albanian friends here," he said. "But if Serbs who have lived here next to Albanians for 100 years cannot stay, I certainly cannot stay."

Correspondents John Ward Anderson in Prizren and Daniel Williams in Klina contributed to this report.