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.
The war crimes trial of ex-Bosnian Serb commander General Ratko Mladic has been delayed by a judge in The Hague due to "errors" by prosecutors in disclosing evidence to defence lawyers.
Alphons Orie told the UN Yugoslav war crimes tribunal today he was delaying the case due to "significant disclosure errors" by prosecutors who are obliged to share all their evidence with Mladic's defence team.
He says judges are still analysing the "scope and full impact" of the error.
ITV News correspondent Paul Davies reported on the issue of Ratko Mladic's arrest warrant in 1996. Today the trial of the former Bosnian Serb general began at a war crimes court in The Hague. He faces 11 war crime charges, including two genocide counts. He denies all the charges.
Bosnian Serb general Ratko Mladic taunted Srebrenica survivors at the start of his trial for genocide, running his hand across his throat in a gesture of defiance to relatives of the worst massacre in Europe since World War Two.
Mladic, now 70, flashed a thumbs-up and clapped his hands as he entered the courtroom in The Hague, where he faces possible life imprisonment for allegedly leading the slaughter of 8,000 unarmed Muslim boys and men in Srebrenica in 1995.
Later, Mladic made eye contact with one of the Muslim women in the audience, running a hand across his throat, in a gesture that led Presiding judge Alphons Orie to hold a brief recess and order an end to "inappropriate interactions."