KUZMIN, Yugoslavia, Aug. 6— After viewing themselves as victors against Muslims and Croats for the past four years, tens of thousands of Serbs found themselves refugees today, a humiliating role reversal for a people proud of their skills as warriors.
Riding tractors, hunched in trailers and astride horses, they poured from Croatia into Bosnia in one of the biggest exoduses of refugees in the fighting since Yugoslavia broke up in 1991. Today, many of them were steadily moving on into Yugoslavia, and some were already showing up in Belgrade, the capital.
About 100,000 Serbs, terrified of reprisals at the hands of Croatian forces, fled towns and villages in the region the Serbs called Krajina just ahead of the advancing Croatian military, the United Nations High Commissioner for Refugees said. United Nations officials in Croatia said they believed the figure was closer to 60,000, however.
The Croatian Government declared victory over the Serbs in Krajina today after a three-day offensive to retake the territory. The Serbs had held the region for four years. [ Page A6. ]
The refugees, many of them armed and some of them still in battle fatigues from the fighting positions they had abandoned, were swirling around western Bosnia from different routes out of Krajina as they sought shelter in several towns, the United Nations agency said.
Many of them brought with them the bitter realization that they were suffering the same kind of ethnic cleansing that the Serbs had carried out, with the winners of territory forcing members of opposing ethnic groups to leave the area.
"I realize that what is happening now to us is a different side of the same coin of ethnic cleansing," said Boris Mijakovac, a 30-year-old psychologist, who said he had fled Friday night from St. Sava hospital in the Krajina capital, Knin, with much of the staff. "But it's a great shock psychologically after it has been built up that we are winners."
It was particularly humbling, Mr. Mijakovac said, that the Yugoslav Army, which had won the 1991 war that created the secessionist Krajina as a symbol of Serb strength, had failed to come to its defense. President Slobodan Milosevic of Serbia, the most powerful leader remaining in Yugoslavia, has been seeking an end to international sanctions by distancing himself from rebel Serbs in Croatia and Bosnia.
Earlier, however, both Bosnian and Croatian Serbs had held on to a vision of "Greater Serbia" that would join territory held by both groups with the current republic of Serbia.
"We all expected Serbia to protect us," Mr. Mijakovac said. "Everyone thinks Milosevic has betrayed us.
With his girl friend, who would give her name only as Ivana, and a group of relatives, Mr. Mijakovac was in one of the first cars that crossed the border from Serb-held territory in Bosnia into Serbia at Simska Raca, 60 miles west of Belgrade, early this morning. Many of the cars were jammed with three generations of one family -- grandmothers holding grandchildren and parents driving -- who had fled with only the clothes they had on.
The International Committee of the Red Cross said the number of Serbian refugees from Krajina would easily surpass the 29,000 Muslim refugees who trekked to the United Nations-designated "safe area" of Tuzla after the Bosnian Serbs overran two other United Nations-protected enclaves last month at Srebrenica and Zepa.
"If we end up, as it seems, with more than 100,000 refugees, it's the biggest flow of population since the start of this war," said Josue Ancelmo, a Red Cross official in Belgrade. While many of the refugees from Croatia had long lived in the Krajina region, many of the refugees within Bosnia have had to flee over and over as territory has changed hands over the past three years.
A steady stream of cars was waved through the Serbian border post on the road leading to Belgrade, with a line of vehicles more than six miles long waiting to go through. Guards were instructed to let them pass by the authorities in Belgrade, refugee agency officials said.
But in the Serb-held areas of western Bosnia where refugees with slower methods of transportation were congregating, the atmosphere was tense, aid officials said.
