Former Bosnian Croat General Slobodan Praljak appeared to drink what he said was poison just as his 20-year sentence on war crimes was upheld in The Hague. Praljak then proceeded to drink from the glass and announced that he had taken poison, according to the BBC.
After drinking Praljak told the court: "I am not a war criminal, I oppose this conviction".
The suicide of Slobodan Praljak at the UN-mandated International Criminal Tribunal for the former Yugoslavia (ICTY) is a multifaceted scandal.
Presiding judge Carmel Agius immediately suspended the proceedings and an ambulance was called.
Slobodan Praljak, 72, died in hospital, with the United Nations court announcing that the courtroom was now "a crime scene", the BBC said.
A guard for the court appealed for calm, and confirmed to journalists that Praljak was alive.
Officials said he was "receiving medical treatment" but Croatian state TV later reported that Praljak had died.
Praljak had been convicted of being involved in a campaign by Croat forces to persecute Bosnian Muslims to carve out a Croat ministate. "Defense minister" Bruno Stojic was sentenced to 20 years.
He had launched an appeal over a 2013 verdict against wartime Croatian figures who were sentenced to a total of 111 years in prison for crimes committed in the Bosnian conflict.
The Tribunal was set up in 1993, while fighting still raged in the former Yugoslavia, and has indicted 161 suspects for war crimes and convicted 90 individuals.