Former Yugoslav President Slobodan Milosevic has been found dead in the detention centre at The Hague tribunal.
The tribunal said an autopsy would be conducted to establish cause of death, but there was no indication of suicide.
Zdenko Tomanovic, a lawyer for Mr Milosevic, says the autopsy should take place elsewhere as his client said he was being poisoned in the jail.
Mr Milosevic, 64, had been held at the UN war crimes tribunal for genocide and other war crimes since 2001.
Mr Milosevic suffered from high blood pressure and a heart condition.
Last month the tribunal rejected a request by the former president to go to Russia for medical treatment.
"Russian doctors were prepared to give him the necessary aid and the Russian authorities guaranteed to meet all the demands of the International Criminal Tribunal for the former Yugoslavia," a spokesman from Russia's foreign ministry said on Saturday.
"Unfortunately, in spite of our guarantees, the tribunal did not agree to give Slobodan Milosevic the possibility of being treated in Russia," he added.
The tribunal has ordered an inquiry into the death.
Damage to tribunal
"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 BBC's Geraldine Coughlan at The Hague says Mr Milosevic's death is a blow to prosecutors, who had been hoping to convict him as being part of a joint criminal enterprise that operated across the former Yugoslavia, intent on setting up a greater Serbian state.
Mr Milosevic faced charges of war crimes and crimes against humanity for his alleged central role in the wars in Bosnia, Croatia and Kosovo during the 1990s.
He also faced genocide charges over the 1992-95 Bosnia war, in which 100,000 people died.
The news was met with joy by survivors of Srebrenica
Mothers and widows of Muslims killed in Srebrenica during the Bosnian war said they regretted that Mr Milosevic's death meant he would never face justice for the killings.
"However, it seems that God punished him already," said Hajra Catic of the Association of Srebrenica mothers.
Serbia-Montenegro's Foreign Minister Vuk Draskovic, who accused Mr Milosevic of organising the assassinations of many of his colleagues and family, said it was a pity the former president had not faced justice in Belgrade.
Mr Milosevic's brother Borislav was quoted by Interfax news agency as saying the war crimes court was "entirely responsible" for his death and that the UN could not be trusted to carry out an autopsy.
Mr Milosevic's lawyer Mr Tomanovic said he had made a formal request for the autopsy to be moved to Moscow "having in mind his claims yesterday that he was being poisoned in the jail".
CHARGES AGAINST MILOSEVIC
Genocide, relating to the massacre in Srebrenica, Bosnia
Crimes against humanity, relating to Bosnia, Croatia and Kosovo
Grave breaches of the Geneva Conventions, relating to Bosnia and Croatia
Violations of the laws or customs of war, relating to Bosnia, Croatia and Kosovo
However, the tribunal has refused the request saying an autopsy will be carried out by Dutch pathologists on Sunday morning. Serbian medical experts will also attend.
The tribunal rejected criticism over its handling of Mr Milosevic.
"The International Criminal Tribunal for the former Yugoslavia takes the utmost care of its indictees and of (Milosevic) in particular," spokesman Christian Chartier said. "We cannot be blamed for negligence."
The BBC's Matt Prodger in Belgrade says that Mr Milosevic was not particularly popular at home, but many Serbs are intensely suspicious of The Hague tribunal and do not believe Serb defendants are fairly treated there.
The Serbian government says it also wants answers on how Mr Milosevic died and whether anything could have been done to prevent it.
The former president had been ill for some time, and his trial was interrupted last year because of health problems.
His lawyer told BBC News 24 that Mr Milosevic would not have committed suicide because he wanted to complete his trial, which had been due to restart on 14th March and was scheduled to end in May this year.
"In fact he said to me a few weeks ago: 'I hadn't fought this case for as long as I have with any intention to do any harm to myself, Mr Kay'. And that is why he wanted medical treatment," Steven Kay said.
Both the former Serbian leader's parents committed suicide.
Mr Milosevic's death comes just six days after a fellow Serb prisoner at The Hague, Milan Babic, committed suicide.
The Croatian Serb leader, who was serving a 13-year prison sentence for crimes against humanity carried out during the 1991-95 war in Croatia, testified against Mr Milosevic in 2002.