After deputy state prosecutor Antun Kvakan read the indictment, Norac and Ademi said they understood what they were charged with but pleaded not guilty to every count.
This is the first case the UN war crimes tribunal in The Hague referred to Croatian courts, two years ago.
Although it proclaimed the 9-17 September 1993 operation which liberated an occupied part of Croatian territory as legitimate, the prosecution accused the two generals of war crimes as commanders.
They are charged with crimes against civilians and excessive and selective shelling of Serb-populated villages, resulting in the destruction of houses and killing of civilians, mainly elderly. The indictment lists by name every civilian and POW that was killed, some allegedly after brutal atrocities.
Kvakan said the highest military and political officials from the 1991-95 wartime period would be called to testify as well as unit commanders from the Lika battlefield to determine who gave them orders and how the operation had been planned.
Ademi was on provisional release pending trial, while Norac was brought from the prison in Lipovica where he is serving a 12-year sentence for war crimes committed in the Gospic area in 1991.
Following the trial are the relatives of the accused, fellow fighters, generals and war veterans, but also representatives of the Organisation for Security and Cooperation in Europe.
The prosecution has announced 137 witnesses, of whom 34 with protected status. Among the witnesses are also UN members who came to the Medak Pocket area after the operation, one of the first liberating operations carried out by the Croatian army.
The defence plans to call about 100 witnesses.