Thousands of Bosnian Serbs jammed roads out of Serb-held suburbs of Sarajevo today, hours before a multiethnic police force -- including Muslims, whom Serbs have fought bitterly for 3 1/2 years -- was scheduled to roll in and take control.

They fled in cars, trucks, horse carts and even sleds, a ragged and hasty exodus along the narrow, ice-slick highway leading east from the suburbs of Vogosca, Ilidza and Ilijas to Serb-held regions beyond.

"We must not wait," said Nebojsa Mocvic, 58, who was leaving his home town of Vogosca. "I know from the television: When the Muslims come in, they'll start killing us."

The guns may be silent around the hills of this Bosnian capital, but a nasty war of words this week helped drain Serb hope and at times fuel hysteria over whether peace has a chance in the suburbs of Sarajevo. The entry of a new police force Friday morning -- theoretically made up of Muslims, Croats and Serbs -- was supposed to be a step on the way to a multiethnic society. Instead, it has prompted more ethnic separation.

The continued flight from the Sarajevo suburbs came on a day of two other major developments: Bosnian President Alija Izetbegovic was hospitalized with an apparent heart ailment, and yet another problem surfaced in negotiations between top Serb military officers and NATO commanders.

Late tonight, government-controlled television reported that Izetbegovic was admitted to Kosevo Hospital in Sarajevo and was under observation after suffering "heart problems" this morning.

Hospital director Fahrudi Konjhodzic said Izetbegovic, 70, should be "spared of all duties," according to the television report. It was unclear, however, how the government was reacting to that advice. Bosnian officials met in private and "there was no word on who would take over" his duties, state television reported.

A senior party official told state radio that Izetbegovic's life was not in danger. "It is not life-threatening. There is no reason for any concern," said Edhem Bicakcic, vice president of the ruling Party of Democratic Action.

Meanwhile, for the second time this week -- just days after a summit in Rome was credited with easing tensions that threatened the Dayton peace accord -- Serb military leaders resisted meeting with NATO commanders.

NATO spokesmen today said the Serb military was being given another 48 hours to begin high-level contact with the NATO-led peace force and to agree to meet with Croat and Muslim commanders in NATO-sponsored talks. NATO's concern prompted its commanders to back off from a report to the U.N. Security Council that the general terms of the peace agreement were being honored.

The NATO assessment is a key to whether economic sanctions against the Bosnian Serbs are suspended. British Lt. Gen. Michael Walker, the commander of NATO ground troops in Bosnia, said cooperation would be tested today and Friday. If the Serbs do not establish contacts, he added, "it will damage any decision about the sanctions."

The day's most vivid scenes, however, took place in the Sarajevo suburbs. Between Friday and March 19, five Serb-held suburbs are scheduled to fall under control of a new joint Muslim-Croat federation. Since it was announced Monday that joint police patrols would begin in Vogosca on Friday, the Bosnian Serb media have pounded out a message of fear to the 40,000 to 50,000 Serbs living around Sarajevo.

Attempts by U.N. officials to restore calm were unsuccessful. "We're trying to combat what effectively is an attempt by the Serb government to empty the suburbs. . . . There's a propaganda hype in Pale {the Serb stronghold} media, and the show masters of this whole operation want these people to leave. It's a fairly cynical political manipulation," said Kris Janowski, spokesman for the U.N. High Commissioner for Refugees.

By late today, Vogosca residents had no assurances that fellow Serbs would, in fact, be part of the new 44-member police force. U.N. officials, who earlier this week had said no Serbs had applied for the force, said they had "some indication that Serbs want to apply" -- but still no applicants.

Effective pressure from Pale on the media and Serb communities helped to keep Serbs from applying, aid workers said. Many Serb officers who wanted to serve were too intimidated to do so, they said.

"We can't force people to apply," said Jens Dunweber, a coordinator for the International Police Task Force, which will monitor the new joint police. "The problem is how do we encourage people to join and to stay as part of the community?"

Some Serb civic leaders who have been advising the civilian coordinators of the peace accord expressed frustration this week with the timing of the changeover and the lack of grass-roots planning, notably by the High Representative's Office headed by diplomat Carl Bildt.

"There are good honest people who worked in the police before the war and who are professionals and want to work again," said Rade Vuletic, who has acted as a liaison between Sarajevo planners and suburban Serbs. "But what has happened this week is chaos. If more isn't done to convince people to stay, they will all leave."

An estimated 6,000 Serbs were living in Vogosca before Monday's announcement about the new police force. This afternoon, Olja Markovic, an aid worker registering people who opted to stay, said that so far only 60 people had come to her office in city hall -- a building that was ransacked during the exodus this week -- asking for help.

"They are all confused," she said. "They don't know if it's better to go or to stay."

"I don't want to live without the Serb police," one 67-year-old man named Mila said this afternoon, asking for advice from strangers but too frightened to tell them his last name. "All of them left two hours ago." CAPTION: With horses pulling their belongings in a cart, Bosnian Serbs slog through the slush of a mountain road as they leave the Sarajevo suburb of Vogosca. CAPTION: Bosnian Serb refugees carry their belongings as they walk along a road out of Vogosca, hours before a new multiethnic police force was to start patrolling.