Milosevic, 64, apparently died of natural causes, the U.N. tribunal said. He was found dead in his bed at the U.N. detention center, but it was unclear exactly when he had died. "The guard immediately alerted the detention unit officer in command and the medical officer. The latter confirmed that Slobodan Milosevic was dead," the tribunal said in a statement.
The tribunal said all prisoners at the detention center near The Hague are routinely checked every half hour, but it had no more information. Milosevic has been on trial since February 2002, defending himself against 66 counts of crimes, including genocide, in Croatia, Bosnia and Kosovo.
The prosecution claimed Milosevic orchestrated a brutal campaign of ethnic cleansing against non-Serbs during the collapse of the Yugoslav federation in an attempt to link Serbia with Serb-dominated areas of Croatia and Bosnia to create a new Greater Serbia.
But the trial repeatedly was interrupted by Milosevic's poor health and chronic heart condition.
It was recessed last week to await his next defense witness. Milosevic also was waiting for a court decision on his request to subpoena former President Bill Clinton as a witness.
He also recently asked the tribunal to be released temporarily to go to Moscow for treatment at a heart clinic. The tribunal rejected the request, fearing he would not return to complete his trial.
Steven Kay, a British attorney assigned to represent Milosevic, said Saturday that the former Serb leader would not have fled, and was not suicidal.
"He said to me: 'I haven't taken on all this work just to walk away from it and not come back. I want to see this case through,'" Kay told the British Broadcasting Corp.
Milosevic's death comes less than a week after the star witness in his trial, former Croatian Serb leader Milan Babic, was found dead in the same prison. Babic, who was serving a 13-year prison sentence, committed suicide.
His testimony in 2002 described a political and military command structure headed by Milosevic in Belgrade that operated behind the scenes.
Milosevic's death will be a crushing blow to the tribunal and to those who were looking to establish an authoritative historical record of the Balkan wars.
Though the witness testimony is on public record, history will be denied the judgment of a panel of legal experts weighing the evidence of his personal guilt and the story of his regime.
"Unfortunately, he did not face justice for crimes he has committed in Kosovo as well," Kosovo's Deputy Prime Minister Lufi Haziri said in Pristina.
The European Union said Milosevic's death does not absolve Serbia of responsibility to hand over other war crimes suspects.
The death "does not alter in any way the need to come to terms with the legacy of the Balkan wars," Austrian Foreign Minister Ursula Plassnik, whose country holds the rotating EU president, said in Salzburg.
Milosevic was due to complete his defense at the war crimes tribunal this summer.
A figure of beguiling charm and cunning ruthlessness, Milosevic was a master tactician who turned his country's defeats into personal victories and held onto power for 13 years despite losing four wars that shattered his nation and impoverished his people.
Milosevic led Serbia, the dominant Yugoslav republic, into four Balkan wars during the 1990s. The secret of his survival was his uncanny ability to exploit what less adroit figures would consider a fatal blow.
Each time he would bounce back, skillfully reinventing himself in a series of political transformations — as a devout communist, a reform-minded nationalist, and again as a communist at a time when most of the world had abandoned Marxist ideology.
He once described himself as the "Ayatollah Khomeini of Serbia," assuring his prime minister, Milan Panic, that "the Serbs will follow me no matter what." For years, they did — through wars which dismembered Yugoslavia and plunged what was left of the country into social, political, moral and economic ruin.
But in the end, his people abandoned him: first in October 2000, when he was unable to convince the majority of Yugoslavs that he had staved off electoral defeat by his successor, Vojislav Kostunica, and again on April 1, 2001, when he surrendered after a 26-hour standoff to face criminal charges stemming from his ruinous rule.
Milosevic was born Aug. 20, 1941, in Pozarevac, a drab factory town in central Serbia best known as the home of one of the country's most notorious prisons.
His father was a defrocked Orthodox priest and sometime teacher of Russian. His mother was also a teacher. Both parents eventually committed suicide.
In high school, he met his future wife, Mirjana Markovic, the daughter of a wartime communist partisan hero. She was also the niece of Davorjanka Paunovic, private secretary and mistress of Josip Broz Tito, the communist guerrilla leader who seized power in Yugoslavia at the end of World War II.
Markovic, who was often accused of being the power behind the scenes during her husband's autocratic rule in the 1990s, also faced corruption charges in Belgrade and has been in self-imposed exile in Russia since 2003.
Milosevic became president of Serbia in 1989 elections widely considered rigged. His rise alarmed the other peoples of former Yugoslavia — Slovenes, Croats, Macedonians, Albanians and others — who feared that the hard-line nationalist would allow Serbs to dominate the country.
In 1991, Croatia and Slovenia declared their independence from Yugoslavia. Milosevic sent tanks to Slovenian borders, triggering a brief war that ended in Slovenia's secession.
Serbs in Croatia, encouraged by Milosevic, took up arms. Milosevic responded by sending the Serb-led Yugoslav army to intervene, triggering a conflict that left at least 10,000 people dead and hundreds of Croatian villages and towns devastated before a U.N.-patrolled cease-fire was arranged in January 1992.
Three months later, Bosnia-Herzegovina declared its independence, too. Milosevic bankrolled the Bosnian Serb rebellion, triggering an even bigger war that killed an estimated 200,000 people before a U.S.-brokered peace agreement was reached at Dayton, Ohio, in 1995.
During those conflicts, Yugoslavia was ostracized worldwide and the United States called Milosevic "The Butcher of the Balkans." Strict international sanctions and government mismanagement devastated the economy and left its people impoverished.
Realizing that the conflicts could not continue, Milosevic agreed to the Dayton talks, accepting a deal that abandoned Croatia's rebel Serbs, who were driven from their homes when the Croatian army recaptured almost all the land the Serbs had seized there in 1991.
The Dayton agreement also meant giving up the nationalist goal of a Serb state in Bosnia. Nevertheless, it bought Milosevic time and transformed his image from Balkan villain to benign peacemaker.
Milosevic's term as Serbian president ended in 1997 and the constitution prevented him from running again. However, he exploited legal loopholes in the constitution to have parliament name him president of Yugoslavia, which by then included only the republics of Serbia and Montenegro.
It was the thorny problem of Kosovo, the majority Albanian province that had served as his springboard to power, which finally set the stage for his downfall. In February 1998, Milosevic sent troops to crush an ethnic Albanian uprising there.
The United States and its allies responded by imposing some of the sanctions that were lifted after the Bosnian war. In 1999, after Milosevic refused to sign a Western-dictated peace agreement at Rambouillet, France, NATO launched 78 days of punishing air strikes against Yugoslavia.
Milosevic refused to back down and instead ordered his troops to crack down on Kosovo Albanians even harder. More than 800,000 Albanians fled into neighboring Albania, Montenegro and Macedonia before Milosevic finally accepted a peace plan and handed over the province to the United Nations and NATO in June 1999.
Before the conflict ended, the U.N. tribunal indicted Milosevic and four of his top aides for war crimes and crimes against humanity allegedly committed in Kosovo. Milosevic became the first sitting head of state ever to be indicted for such crimes. Later, they broadened the charges against him to include genocide.