THE HAGUE - Dutch prosecutors were investigating Thursday how a Bosnian Croat war criminal managed to commit suicide in front of shocked UN judges, in scenes set to cast a shadow over the court's two-decade legacy.

Staff at the imposing buildings of the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague had been expecting to spend the next few weeks quietly winding down the court which closes on December 31, after the judges delivered their final ruling on Wednesday. But courtroom one is now an active crime scene after the dramatic events when former Bosnian Croat military leader Slobodan Praljak tipped the contents of a small brown bottle into his mouth and died shortly afterwards in hospital.

An autopsy will be carried out shortly as a matter of the "highest priority," Frans Zonneveld, spokesman for the Dutch prosecution in The Hague, told AFP.

He revealed that tests had found that the bottle contained a "chemical product which can cause death".

The unprecedented gesture came as Praljak, 72, angrily denied being a criminal, after judges rejected his appeal and upheld his 20-year jail term for atrocities committed in a breakaway Bosnian Croat statelet during the 1990s wars.

Dutch prosecutors said in a statement late Wednesday that the investigation into his death "for the time being the inquiry will focus on assisted suicide and violation of the Medicines Act."

It remains a mystery exactly what Praljak drank and how he managed to evade tight security to smuggle the bottle into the tribunal.

There are also questions about how he acquired the substance and if it was in the fortress-like UN detention centre in The Hague where he was being held.

Under court rules, everyone entering the detention centre is subjected to tight security "irrespective of his or her status, nationality, function or age".

Everyone must pass through security scanners, and there may also be "a search of clothing."

Defendants are allowed to have access to their medications in the detention centre, to be administered under the supervision of the chief medical officer.

And they are also allowed approved visits from personal doctors.

The drama came as seasoned judges, many of whom have worked for a long time at the court set up in 1993, handed down their very last verdict in the appeal case of six Bosnian Croat political and military leaders.

They upheld all the five other jail terms ranging from 25 to 10 years sentences.

Praljak's death caused shockwaves in Croatia, where for some he had been considered a hero.

But Croatian President Kolinda Grabar-Kitarovic said Thursday that citizens had to admit crimes had been committed by fellow Croats in Bosnia.

"We Croats need to have the strength to admit that some of our fellow compatriots in Bosnia committed crimes and they have to be held responsible for them," she added.

After candles were lit overnight for the former general in Mostar, once the capital of the breakaway Herzog-Bosna, Croatian lawmakers held a minute's silence Thursday to remember all victims of the 1990s conflicts.

"All victims have to remain forever in our collective consciousness and yesterday's death of General Praljak should remain the last act of the tragic events of war," said parliamentary speaker Gordan Jandrokovic.

Croatian Prime Minister Andrej Plenkovic repeated his conviction that the verdict - which found all six men to be key participants in a scheme to ethnically cleanse Bosnian Muslims from the area - was "unjust".

But he said the UN court had tried "individuals and not states, and it does not speak about responsibility of a state".

"These men knew how manipulate power and to ensure everyone is watching," said Frederiek de Vlaming, an expert in international law at the University of Amsterdam. "It's a kind of protest which we haven't seen before," she told the De Volkskrant newspaper, adding that it "illustrates how the suspects in the region view the court".