Three weeks ago, Bugar Dugolli called a human rights lawyer in Belgrade and asked him to visit his 37-year-old brother Enver, a prominent Kosovo Albanian political prisoner, who was jailed in the north Serbian town of Sremska Mitrovica.

But when the lawyer, Hysni Bytyci, arrived at the prison, he was told that all ethnic Albanian prisoners, including Enver Dugolli and as many as 100 others, had been transferred. The warden provided no details but suggested they had been taken to the southern city of Nis, where dozens of other ethnic Albanian dissidents also were being held.

When Bytyci contacted prison authorities in Nis, he was told they had received no transfers from Sremska Mitrovica, but they said the prisoners might have been taken to any one of a number of prisons in Kosovo. The lawyer, fearing for his own safety, then gave up the search and has asked the families of prisoners not to contact him.

"I'm very concerned," said Bugar Dugolli, 28, who was president of the Students Independent Union at the University of Pristina before he fled Kosovo. "I live with the hope that he is okay. But all those prisoners are in grave danger."

In recent weeks, Serbian authorities have released 2,500 prisoners who were swept into jails in Kosovo in the weeks since NATO's air campaign against Yugoslavia began on March 24. But prominently missing are leading political prisoners who were jailed in the last 10 years because of their active support for Kosovo's independence from Serbia, the dominant Yugoslav republic.

At least 2,000 ethnic Albanians were imprisoned on political charges before the airstrikes began, according to human rights organizations, and their fate remains unknown. But their families and human rights groups fear that the prisoners' resistance to rule by Belgrade makes them among the most vulnerable ethnic Albanians still in Serbia; many are part of the leadership of the Kosovo independence movement.

The concern is compounded by the harsh treatment, including documented cases of torture, that many endured even before the NATO air campaign began. And it is fueled further by reports from some former prisoners who have been allowed to cross into neighboring Macedonia or Albania in the last two weeks that conditions in the prisons are hellish.

"My children are very sad and are asking when are we going to see him," said Shahadije Xhemajli, 38, whose husband, Bajrush Xhemajli, a member of the separatist Kosovo Albanian government, was given a six-year prison sentence in 1993 for his activities. "We don't know where Bajrush is, and no one can help us."

Ilir Ruzhdi's brother, Blerim Olloni, an attorney, was arrested in November 1994 and accused of forming an ethnic Albanian police force to rival the official Serbian force. Olloni was taken in for the three days of investigative detention as allowed by Yugoslav law but was held for six months. His wife said police phoned her from the detention center so she could listen as they beat her husband.

In July 1995, Olloni, 42, was sentenced to a six-year prison term, and as of March he was being held at Dubrova prison outside the Kosovo city of Mitrovica. The prison was bombed in April by NATO, which said it was being used by the Yugoslav army as a command center. Nineteen prisoners reportedly were killed, but the families of prisoners at the facility, including the Ollonis, do not know if their relatives are among the dead.

Among the prisoners reportedly being held at Lipljan prison, 10 miles south of Pristina, the Kosovo capital, is Albin Kurti, an ethnic Albanian official of the Students Independent Union of the University of Pristina and a former aide to Adem Demaci, spokesman for the insurgent Kosovo Liberation Army. Kurti's father is also imprisoned there and possibly several other relatives, according to several men in a group of 61 released from the prison last week.

The prison is notorious as a place where ethnic Albanians are brutally beaten. Last autumn, the Belgrade-based Humanitarian Law Center reported, for example, that "detainees . . . are abused on a daily basis" while moving from one part of the prison to another.

Several former inmates, who were inexplicably released and bused to the Macedonian border last Thursday, said they had been forced to sit cross-legged on the floor of a bare cement room for hours at a time with their hands behind their backs and their heads bowed. Through the windows, they could hear screams, mostly from new arrivals, as well as antiaircraft fire at NATO warplanes.

Each day, they were awakened at 6 a.m. At 11 a.m. and 4 p.m., they were given stale bread and a small amount of vinegar and pepper to eat. They remembered receiving a spoonful of marmalade each for their bread three times over the past month.

"Why don't you accept the policy of Milosevic?" asked one policeman as he struck a prisoner, according to accounts from the former inmates. "This is a present from Clinton," said another policeman as he struck one of the prisoners on the head with an iron rod, leaving a large wound that required stitches. An ex-prisoner named Isret said police asked, "What are the weapons you are using? You gave money to the KLA. How many family members do you have? How many are in the KLA?"

"I kept saying I didn't know anyone, but it didn't matter what answers we gave," Isret said.

Those who had difficulty speaking Serbian also were beaten, said Astrid van Genderin Stort, a spokeswoman for the U.N. High Commissioner for Refugees at the Blace refugee camp in Macedonia. She said some of the men told her in interviews of being forced to hold their hands over their heads and pass through a gantlet of policemen who beat them. Others told of being forced to kneel on the floor and of being kicked repeatedly.

When the men relieved themselves, their urine was often bloody from internal hemorrhaging, and at least one of the men in their room -- a professor from the Kosovo city of Urosevac -- died of his injuries, former prisoners said.

A number of those who entered Albania after being released have contracted tuberculosis, or the disease reemerged from a dormant state, probably because of their imprisonment, according to doctors here who examined them. All the former prisoners are being screened for the disease.

"These are the worst prison conditions I've heard of since the tiger cages of Vietnam," said David Langness, a former medic in Vietnam who treated American POWs and who visited Kosovo refugees in Albania this week with a UCLA medical team. He said prisoners were crammed 60 to a cell and that most of them had chronic diarrhea.

According to Bilal Sherifi, an adviser to Hashim Thaqi, political leader of the Kosovo Liberation Army, the guerrilla group has received unconfirmed reports that in the first days of NATO's bombing a small number of prominent KLA prisoners signed their release papers, were freed and then picked up again by special police just outside the prisons where they had been held. Sherifi said the KLA did not know where the prisoners were taken, but he said he fears the worst.

Most of the prisoners in Lipljan were forced to give confessions, reciting aloud, "I am a terrorist" and acknowledging that they were being held with justification under the Yugoslav penal code.

Bugar Dugolli last saw his brother in January. The prisoners had just won the right, after the intervention of the International Committee of the Red Cross, to speak Albanian to family visitors. Until the Red Cross intervened, the prisoners were beaten if they did not speak Serbian, human rights groups said. Dugolli said his brother tried to be cheerful during such visits for the sake of his wife and two children. But two years of ill treatment had begun to take its toll. He said he does not like to imagine how the prisoners were treated after NATO started bombing.

"It's very hard to think about what might be happening to your brother," Dugolli said. "But I will be happy, and I know my brother will be happy, if the children of my brother have the opportunity to live in freedom in Kosovo."

Finn reported from Tirana; Smith from Skojpe, Macedonia. Correspondent John Ward Anderson in Kukes, Albania, contributed to this report.

CAPTION: Ethnic Albanian men held prisoner by Yugoslav troops for weeks cross into Albania at Morina. About 200 former prisoners crossed the border last week.