To continue with the trial would place the Bosnian Serb military chief at risk of a “miscarriage of justice”.
Ratko Mladic, 78, dubbed the “Butcher of Bosnia”, is challenging his 2017 conviction and life sentence for genocide and crimes against humanity.
He is also challenging his conviction for war crimes in the civil war in former Yugoslavia including the 1995 Srebrenica massacre.
But Mladic's lawyers said they were taking part under protest in the two-day hearing in The Hague, after judges earlier this week rejected a bid to postpone it pending a fresh medical assessment.
A frail-looking Mladic appeared in court wearing a mask because of coronavirus regulations, which he later removed.
Wearing a suit with his red tie askew, he initially complained that he could not follow the hearing through headphones.
"This hearing today is inappropriate and threatens... a miscarriage of justice, such that I cannot abide to be a silent participant in this questionable proceeding," defense lawyer Dragan Ivetic told the court in opening remarks.