Former Yugoslav President Slobodan Milosevic has died, the United Nations criminal tribunal in the Netherlands announced on Saturday.
"Milosevic was found lifeless on his bed in his cell at the United Nations detention unit," the tribunal said in a statement.
"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 the Dutch police and a Dutch coroner were called in and started an inquiry. A full autopsy and toxicological examination have been ordered. Milosevic's family has been informed, it added.
Milosevic, 64, suffered a heart condition and high blood pressure which had repeatedly interrupted his trial in the Hague on charges of genocide, crimes against humanity and war crimes during the bloody disintegration of Yugoslavia in the 1990s.
French Foreign Minister Philippe Douste-Blazy said in the Austrian city of Salzburg that Milosevic had died of natural causes.
In a statement at an EU foreign ministers meeting just after news of Milosevic's death was confirmed, he said: "With the death of Milosevic, one of the main actors if not the the main actor in the Balkan wars of the late 20th century has left the scene."
"I would like to spare a thought for all those who suffered so much from ethnic cleansing, tens of thousands of men, women and children, which Milosevic conceived and planned."
The Hague tribunal's chief prosecutor, Carla Del Ponte of Switzerland, said she deeply regretted what had happened because the trial had been winding up. She added there had been enough evidence for a conviction.
"I find it regrettable also for all the victims, the thousands of victims, who were waiting for justice," she told German-language Swiss television.
Del Ponte said the result of an autopsy expected in the coming days would determine the cause of death.
The Swiss foreign ministry has not issued a statement on Milosevic's death but spokeswoman Carine Carey told swissinfo: "The process of reconciliation in the Balkans must continue to be dealt with by the tribunals."
"They are the ones to bring the people accountable for the events to justice."
Two cardiologists treating Milosevic in The Hague had warned he was at risk of a potentially life threatening condition known as a hypertensive emergency, when surges in blood pressure can damage the heart, kidneys and central nervous system.
Last month, the tribunal rejected a request by Milosevic to travel to Russia for specialist medical treatment, noting that his trial - that has already lasted four years - was in the final stages and he might not return to complete it.
The court said Milosevic's lawyers had not shown that his medical needs could not be met in the Netherlands and said experts from abroad could come to The Hague to treat him.
Milosevic, who was overthrown in 2000 and sent to The Hague in June 2001, said last month his health was worsening.
The former Serb strongman was defending himself and had refused to cooperate with court-appointed lawyers who were on standby to fill in for him when he was ill.
He had used up more than four-fifths of the 150 days allotted for his defence, suggesting the case could be wrapped up in the next few months barring any new delays. Judges would then need several months to deliberate before a verdict.
Milosevic was charged with 66 counts of genocide, crimes against humanity and war crimes in complex indictments covering conflicts in Bosnia, Croatia and Kosovo in the 1990s. He declined to enter a plea.
Last week, former rebel Croatian Serb leader Milan Babic committed suicide at the tribunal's detention centre. Babic had testified against Milosevic and was in The Hague to appear in the trial of another top Croatian Serb.
swissinfo with agencies
Slobodan Milosevic was born in Pozarevac, Serbia, on August 20, 1941, son of a theologian.
His father and mother both committed suicide.
A communist functionary, he elbowed his way to the top of Yugoslav politics in the power vacuum left by the 1980 death of Yugoslav dictator Josip Broz Tito.
He was toppled on October 5, 2000, still resisting the result of a presidential election, when huge protests erupted in Belgrade.
Milosevic was arrested six months later and transferred in June 2001 to the United Nations war crimes tribunal in The Hague.
The International Criminal Tribunal for the Former Yugoslavia was established by Resolution 827 of the United Nations Security Council in May 1993.
Based in The Hague, it is the first international body for the prosecution of war crimes since the Nuremberg and Tokyo trials held after the Second World War.
The tribunal has jurisdiction over individuals responsible for genocide, war crimes and crimes against humanity in the territory of former Yugoslavia after Jan. 1, 1991.
Switzerland's Carla Del Ponte of Switzerland is the tribunal's chief prosecutor.
The tribunal may not try suspects in absentia, nor impose the death penalty. The maximum sentence is life imprisonment.
Two of the most wanted men by the tribunal are Radovan Karadzic and Ratko Mladic - the fugitive wartime leader of the Bosnian Serbs and his military chief.