Croatian state TV said Slobodan Praljak, 72, a former commander in Bosnia's 1992-95 war, appeared to drink from a small bottle moments after judges at the worldwide criminal tribunal in The Hague reconfirmed a 20-year sentence.
A screengrab purporting to show wartime commander of Bosnian Croat forces Slobodan Praljak drinking what he said was poison in court.
A Bosnian Croat war criminal has claimed to take poison while in court.
"Don't take away the glass", Agius said, instructing the guards to lower blinds and block a glass-partition separating the court from the public. "I am not a war criminal". "Judges, Slobodan Praljak is not a war criminal!" he said after the decision as announced. "I oppose this conviction".
The judge Carmel Agius called for a doctor and stopped the trial.
Dutch police will not comment on the TV report based on "sources close to Gen. Praljak".
"Mr Praljak was immediately assisted by the ICTY medical staff".
Agius, a former judge at the Maltese courts and United Nations representative of the government, was elected to the role of president of the International Criminal Tribunal for the Former Yugoslavia in 2015.
Praljak was specifically charged with ordering the destruction of Mostar's 16th-century bridge in November 1993, which judges said "caused disproportionate damage to the Muslim civilian population".
Although they were allies against the Bosnian Serbs in the Bosnian War of 1992 - 1995, Bosnian Croats and Muslims were also in conflict for nearly a year, with Mostar seeing some of the bloodiest conflict as the country of Yugoslavia messily divided.
The ICTY, established in 1993 by the UN Security Council, indicted 161 war crimes suspects from Bosnia, Croatia, Serbia, Montenegro and Kosovo.
The appeals hearing comes a week after the judges sentenced former Bosnian Serb military commander Ratko Mladic to life in prison.