Slobodan Praljak, 72, died in hospital, with the UN court announcing that the courtroom was now "a crime scene", the BBC said.
On hearing that his 20-year jail term had been upheld, the ex-commander of Bosnian Croat forces said he "was not a criminal" and then drank from a bottle.
In 2013, he was sentenced for crimes in the city of Mostar during the Bosnian war from 1992-95.
Praljak was one of six former Bosnian Croat political and military leaders up before the International Criminal Tribunal for the former Yugoslavia (ICTY).
They were attending the final appeals judgment to be handed down by the court, the BBC report said.
Though allies against the Bosnian Serbs in the war, Bosnian Croats and Muslims also fought each other for a period of 11 months, with Mostar seeing some of the fiercest fighting.
After hearing the verdict, General Praljak stood and raised his hand to his mouth, tipped his head back and appeared to swallow a glass of liquid.
"I have taken poison," he said.
Presiding judge Carmel Agius immediately suspended the proceedings and an ambulance was called. "Okay," the judge said. "We suspend the... We suspend... Please, the curtains. Don't take away the glass that he used when he drank something."
An ambulance could later be seen arriving outside the tribunal while a helicopter hovered above the scene. Several emergency rescue workers also rushed into the building carrying equipment in backpacks.
The BBC quoted a ICTY statement as saying Praljak "was immediately assisted by the ICTY medical staff". It said Praljak "was transported to a nearby hospital to receive further medical assistance where he passed away".
An independent investigation is now under way.
(This story has not been edited by Social News XYZ staff and is auto-generated from a syndicated feed.)
Doraiah Chowdary Vundavally is a Software engineer at VTech . He is the news editor of SocialNews.XYZ and Freelance writer-contributes Telugu and English Columns on Films, Politics, and Gossips. He is the primary contributor for South Cinema Section of SocialNews.XYZ. His mission is to help to develop SocialNews.XYZ into a News website that has no bias or judgement towards any.
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