"I do not recognise this hate court. It is a satanic court," Mladic told the International Criminal Tribunal for the former Yugoslavia (ICTY) in The Hague, nevertheless then taking the oath and asking for his false teeth, a request met with laughter from the public gallery.
"Could the security people please bring my teeth so that I can speak better?" asked Mladic, in an apparent attempt to ridicule the court by the man known as the "Butcher of Bosnia" for his alleged role in the Srebrenica massacre.
The court was subsequently adjourned for 20 minutes so that Mladic's dentures could be found.
Both Mladic and Karadzic have been charged with genocide, war crimes and crimes against humanity over their roles in the 1992-1995 Bosnian war, in which 100,000 people died.
Karadzic hopes that Mladic will testify that they did not agree or plan to expel Muslims or Croats from areas under Serb control.
A thin-looking Mladic appeared in court wearing a grey suit, smiling at the audience as he walked in and then at the carefully coiffed Karadzic.
The court had to subpoena Mladic to force him to testify as a defence witness in Karadzic's trial, with his lawyer Branko Lukic on Tuesday making one final attempt to prevent him having to speak.
"We believe Mr Mladic is unfit for testimony," Lukic said.
People with "deception of memory", "they make up facts and believe those facts are the truth," he said.
"He is not able to concentrate on a single document."
The 68-year-old Karadzic is accused of authorising so-called "ethnic cleansing" in the bitter war against Bosnia's Muslim-led government.
Mladic, 71, had refused to testify, citing his health, the amount of time taken up by his own trial and the fact that he might incriminate himself.
But judges rejected Mladic's argument, saying he was in a unique position to tell the court what information he had given the accused in relation to incidents he was allegedly involved in.
The ICTY was created in 1993 to try perpetrators of war crimes committed during the former Yugoslavia's bloody break-up.
Karadzic and Mladic both face charges over the massacre of almost 8,000 Muslim men and boys in the eastern town of Srebrenica in July 1995.
They could have been tried together had they been arrested around the same time. But Karadzic was arrested in July 2008 and Mladic in May 2011.
