Former Bosnian Serb paramilitary commander Milan Lukic has pleaded not guilty at the Hague international tribunal to crimes against humanity.
Lukic was sentenced to 20 years in absentia by a Belgrade court
He was handed over to the tribunal after being extradited from Argentina where he was arrested last August after almost seven years on the run.
The indictment says he led a group called the "Avengers" or "White Eagles" in the 1992-95 Bosnian War.
He is accused of killing more than 100 Bosnian Muslims.
Mr Lukic pleaded not guilty to 21 counts brought by the tribunal.
They include charges of mass executions, murder, persecution and barricading Bosnian Muslim women and children inside houses that were later burnt.
Prosecutors say the White Eagles carried out a campaign of ethnic cleansing in the Bosnian city of Visegrad.
Mr Lukic is indicted along with his cousin, Sredoje Lukic, who is also awaiting trial in the tribunal's detention unit.
Mr Lukic was sentenced in absentia in Belgrade in 2003 to 20 years in prison for the murder of 16 Muslims in Serbia in 1992.