In a historic courtroom reunion of the two alleged chief architects of Serb atrocities during Bosnia's 1992-95 war, Ratko Mladic was testifying under subpoena for his former political master, Radovan Karadzic.
Both men are on trial separately in The Hague for crimes including genocide. They both insist they are innocent.
Mr Karadzic's team said former Bosnian Serb army commander General Mladic was "the one person in the whole world who knows best what happened in the war in Bosnia."
Mr Mladic refused to testify, calling the tribunal a "satanic court."
Former Bosnian Serb army commander General Ratko Mladic condemned the United Nations' Yugoslav war crimes tribunal in the Netherlands as a "satanic court" as he began giving evidence as a defence witness for his former political master, Radovan Karadzic.
Ratko Mladic and Radovan Karadzic are facing charges for war crimes and crimes against humanity.
This is the first time they are appearing in public together since the end of the conflict in former Yugoslavia.
Mr Mladic's request not to testify due to poor health and possible prejudice was denied by the court.
The discovery of a mass grave in northern Bosnia and Herzegovina has brought hope that more evidence could be found for the trial of Ratko Mladic and Radovan Karadzic who are facing trials at the War Crimes Tribunal in The Hague.
Eldar Jahic from the prosecutor's office has told ITV News that teams from The Hague have already visited the site.
Former Bosnian Serb leader Radovan Karadzicwill begin his defence against charges of war crimes at The Hague.
He faces 10 charges including genocide, war crimes and crimes against humanity. He has denied the charges, which relate to the Bosnian war of the 1990s. Karadzic could face a maximum sentence of life in prison if convicted.