The former speaker of the Bosnian Serb parliament, Momcilo Krajisnik, has gone on trial on war crimes charges.
Momcilo Krajisnik may face life imprisonment if found guilty
Mr Krajisnik is charged with genocide and masterminding a campaign of ethnic cleansing against non-Serbs during the Bosnian conflict from 1992 to 1995.
The ally of wartime leader Radovan Karadzic, denies the charges.
Meanwhile, chief UN prosecutor Carla Del Ponte has told Bosnian TV that Nato-led troops missed catching Mr Karadzic by just two hours last month.
Mr Krajisnik is one of the most senior Bosnian Serb officials to face accusations at the International War Crimes Tribunal in The Hague.
His trial is expected to take at least two years and, if convicted he could face a life sentence.
The 58-year-old was speaker of the Bosnian parliament during the war, in which more than 200,000 people died.
He is facing charges including organising detention camps, inciting ethnic hatred and shared responsibility for killing thousands.
"Along with Radovan Karadzic, it was his hand which held firmly the levers of power and authority," prosecutor Mark Harmon said in his opening remarks on Tuesday.
The prosecution says that because of Mr Krajisnik's position in the Bosnian Serb leadership, he had authority over the Bosnian Serb forces as well as the political and governmental sections that participated in the alleged crimes.
He was indicted originally along with former Bosnian Serb President Biljana Plavsic, who pleaded guilty to persecution and was last year sentenced to 11 years in prison.
In her plea, Plavsic named Mr Krajisnik and the two most-wanted Bosnian Serb fugitives - Mr Karadzic and military leader Ratko Mladic - as the architects of the campaign to drive non-Serbs out of Serb-dominated areas in Bosnia.
Mr Karadzic is indicted by the UN court for genocide, but has been on the run for several years.
Carla Del Ponte said an operation by the Nato-led stabilisation force (S-For) stormed the city of Pale on 13 January believing Mr Karadzic to be ill and seeking medical treatment there.
She told Bosnian television the 58-year-old "was there for several hours, but S-For arrived two hours too late".
The prosecutor said she had "no doubt that S-For is doing everything possible" to arrest him before the end of the year.
"Eight or nine new indictments" are also to be issued against members of the Muslim and Croat communities in Bosnia, she added.