Standing tall, with a shock of white hair and beard, he then raised a small brown bottle to his lips, and tipped it into his mouth.
More than an hour after the incident, a court guard told Reuters news agency said Praljak was still "being treated".
Croatian PM Andrej Plenkovic said: "We have all unfortunately witnessed his act by which he took his own life".
Judge Agius declared the courtroom to be a crime scene as he restarted the hearing, though gave no further details.
Praljak was one of six former Bosnian Croat political and military leaders due to hear their appeal verdicts on Wednesday.
Presiding Judge Carmel Agius was forced to suspend the hearing with emergency services entering the courtroom. When I first met him it was on the front line in Croatia in 1991, and as soon as the camera crew arrived all hell erupted.
"I have taken poison", he said, according to a court translator.
Among Praljak's charges was the destruction of the iconic 16th-century bridge in the Bosnian city of Mostar in November 1993, which was eventually rebuilt in 2004 but at the time "caused disproportionate damage to the Muslim civilian population", the judges noted in the 2013 verdict. Praljak was accused of ordering the attack, which is said to be a symbol of Bosnia's devastation in the war.
Mostar saw the worst of the Croat-Muslim clashes, with almost 80 per cent of the city's east destroyed in the fighting.
In their ruling, the judges allowed part of Praljak's appeal, saying the bridge had been a legitimate military target during the conflict. They also had overturned some of his convictions, but refused to reduce his overall sentence.
Praljak, 72, was convicted for trying to create an ethnically pure Croatian state during the Bosnian war in the 1990s, which sparked by the breakup of Yugoslavia.
They have not yet passed judgment on the three remaining defendants: Milivoj Petković, 68, Valentin Corić, 61, and Berislav Pušić, 65.
The tribunal, which last week convicted former Bosnian Serb military chief General Ratko Mladic of genocide and other crimes, was set up in 1993, while fighting still raged in the former Yugoslavia.
During the wider 1992-1995 war the breakaway Croat forces, which were backed by Croatia, mostly sided with the Bosniaks against the breakaway Bosnian Serb republic, which was backed by Serbia.
A war-crimes appeal hearing was halted just after the verdict was announced on Wednesday, when the defendant claimed to have taken poison.
Nick Kaufman, an Israeli defense lawyer who used to work as a prosecutor at the tribunal, also said a defendant could find a way to bring in a banned substance.