The tribunal said in a statement that guards had found Mr. Milosevic, 64, dead in his bed, apparently of natural causes, while they were on their regular rounds. But the time of his death was unclear, and the Dutch police and a coroner began a full investigation. An autopsy is scheduled to be performed Sunday in the Netherlands.
Leaders in the region and in Western Europe immediately expressed regret that he would never be convicted for his role in the disintegration of Yugoslavia and in three wars in the region during the 1990's.
Richard Dicker, a director of Human Rights Watch in New York who has often attended the trial, said it was "a terrible setback first and foremost for the victims of horrific crimes in the former Yugoslavia, and because it deprives the tribunal of a chance to render a verdict on his true role."
"That verdict would be a crucial piece of the record of what happened and who is accountable."
He added: "But Milosevic's death does not undo the legacy of the trials completed already and those that will be completed. The four years of this trial are a loss, but it does not undo more than a decade of work in establishing reference points and responsibilities."
Mr. Milosevic had been in poor health for years, and his heart ailment and high blood pressure repeatedly caused delays in his war crimes trial here on charges of genocide, crimes against humanity and war crimes in Bosnia, Croatia and Kosovo. He refused to enter any plea, and he insisted on acting as his own lawyer, only later accepting any help from his court-appointed lawyers and often clashing with their advice. His long courtroom diatribes, often tuned to play well to a Serbian audience back home rather than the court itself, took up whole sessions.
He was the first former head of state to stand trial for genocide before an international tribunal, and its proceedings, which began in February 2002, had already produced the longest war crimes trial in modern history. His death came as the trial was drawing to a close: he was in the final weeks of his defense, and his concluding statement was expected in late April or May. The judges' verdict was expected by the end of this year.
Mr. Milosevic had complained in recent weeks that his health was worsening, and he pressed the court to allow him to seek treatment at the Bakoulev Scientific Center for Cardiovascular Surgery in Moscow, where his wife and son live. But the court denied his request, saying full medical care was available in the Netherlands and any doctor could come to The Hague to treat him. The Russian Foreign Ministry criticized the court's decision on Saturday after Mr. Milosevic's death.
One of Mr. Milosevic's legal advisers, Zdenko Tomanovic, said Saturday that in recent days, Mr. Milosevic said that someone was trying to poison him. Mr. Tomanovic said he wanted the autopsy to take place in Moscow rather than in The Hague.
"I insist on this request, especially bearing in mind Mr. Milosevic's claims that there were attempts to poison him in the prison," he said. "And yesterday, I also informed the Russian Embassy on behalf of Mr. Milosevic about his claims that his health was willfully destroyed."
The tribunal denied the request for an autopsy in Moscow. A tribunal official said Saturday, however, that two pathologists from Serbia and two Dutch doctors would participate in the autopsy here.
Florence Hartmann, the spokeswoman for the chief prosecutor at The Hague, cautioned that it was too early to rule out suicide as a cause of death. "We really have to wait for the autopsy results," she said.
Mr. Tomanovic insisted that Mr. Milosevic did not kill himself. "I saw his body lying in his cell, and I'm sure that's not true," he said.
One of Mr. Milosevic's court-appointed lawyers, Steven Kay, said in an interview with the BBC that he did not think that Mr. Milosevic took his own life. "We were working on the last stretch," he said. "He was determined to see this through."
Mr. Kay added: "He has a history of suicide in his family — both his parents — but as far as he was concerned, his attitude to me was quite the opposite from that. He was determined to keep fighting his case."
The death was the second major blow for the court in a matter of days. On March 5, Milan Babic, a convicted former politician who had testified against Mr. Milosevic and was cooperating with prosecutors in other cases, hanged himself in his holding cell in The Hague.
Since the Security Council created the tribunal in 1993, it has indicted 161 people. Proceedings have been completed against 85 people, and close to 60 are in custody or awaiting proceedings.
But six senior figures, including the Bosnian Serb wartime leader Radovan Karadzic and his top commander, Gen. Ratko Mladic, are still being sought. Carla Del Ponte, the chief prosecutor at The Hague, has complained that cooperation from Serbia, the dominant Yugoslav republic, has stalled.
In a statement released by her office on Saturday, Ms. Del Ponte said: "The death of Slobodan Milosevic a few weeks before the completion of his trial will prevent justice to be done in his case. However, the crimes for which he was accused, including genocide, cannot be left unpunished. There are other senior leaders accused of these crimes, six of them still at large."
Her office said she would hold a news conference on Sunday.
The European Union insisted Saturday that the death in no way relieved Serbia of its responsibility to hand over war crimes suspects. After a meeting of European foreign ministers in Austria, Foreign Minister Ursula Plassnik of Austria warned that Belgrade must continue to aid the effort "to come to terms with the legacy of the Balkan wars," The Associated Press reported.
The Serbian foreign minister, Vuk Draskovic, who was also at the meeting, said his country would continue on its course of reform. "I am not ready to establish a link between the destiny of Milosevic and the destiny of Serbia," he said, noting that Mr. Milosevic had ordered the assassination of many opposition figures, including himself, The A.P. said. He added, "I can say only that it is a pity that he did not face justice."
Across the Balkans, Mr. Milosevic's death prompted strong emotion, from dismay among those who saw themselves as the victims of his repressive rule, to the sorrow of the now dwindling core of Serbian Socialist Party supporters, for whom Mr. Milosevic was still the nominal party leader.
In Vranje, one of the few towns in Serbia still dominated by the Socialist Party, supporters lowered the Serbian flag to half staff and placed black bands across photographs on office walls.
"Too bad for the guy," said Zoran Ivanovic, a 41-year-old laborer who lives in Vranje. "He was a big Serb. Maybe he made a few mistakes, like not accepting Yugoslavia be turned into a confederation, and perhaps avoiding war. But those charges of genocide, that's baseless."
Among those who had worked to bring Mr. Milosevic to trial, there was a profound note of disappointment on Saturday.
Ms. Hartmann, Ms. Del Ponte's spokeswoman, expressed frustration that at least in a legal sense, Mr. Milosevic would not go down in history as a convicted war criminal. "This is bad for proving the real responsibility of Mr. Milosevic," she said. "There is a presumption of innocence, and now we will not get a conviction."
But Richard C. Holbrooke, a former American ambassador to the United Nations who was also a negotiator of the Dayton peace accord for Bosnia, said that amid the disappointment, there was still justice.
"The trial was too long, but the trial was the verdict," he told the BBC on Saturday. "The Serb people came to understand the truth that he was not a nationalist, but an opportunist. A kind of rough and imperfect justice was served."