The war crimes trial of Radovan Karadzic, who led the Bosnian Serbs through a brutal three-year civil war in the early 1990s, is due to begin about 15 months after he was captured in Serbia.
The prosecution and defence will each have one year to present their case.
Karadzic is boycotting the opening in a defiant gesture against what he considers a rush to justice by the UN court prosecuting him in The Hague.
His refusal to show up at the Yugoslav war crimes tribunal is a blow to survivors who hold him responsible for tens of thousands of deaths during the brutal 1992-95 Bosnian war.
The case comes as a relief after the trial of Karadzic's former political mentor Slobodan Milosevic collapsed without a verdict after he died in 2006.
Observers agree that the 64-year-old Karadzic's absence from Courtroom One at the Yugoslav war crimes tribunal should not overshadow the case's significance.
Karadzic's trial is seen as a chance for the tribunal to make amends for Milosevic's ill-fated trial, which dragged on for four years before his fatal heart attack.
Karadzic, 64, also is charged with genocide - one count for the 1995 murder of 8,000 Muslims in Srebrenica and a second for the Bosnian Serb campaign of ethnic cleansing against the country's Muslim and Croat populations.
He faces nine other charges including extermination, persecution and taking peacekeepers hostage. Karadzic has repeatedly refused to enter pleas, but insists he is innocent. He faces a maximum life sentence if convicted at his trial, which is expected to last at least two years.
He is boycotting the hearing to protest at his lack of time to prepare for the trial, saying he needs months more to get ready.