Ex-Bosnian Serb military chief Ratko Mladic refused to testify in support of his one-time political ally Radovan Karadzic who called him as a defense witness at his war crimes trial at The Hague.
Mladic denounced the U.N. war crimes tribunal as "satanic" and said he did not want to incriminate himself.
It was an unusual reunion for the two men who have not been seen together since the 1990s Bosnia war ended.
Mladic, the former general who headed separatist Bosnian Serb forces, and Karadzic, the political leader, are both accused of responsibility for the massacre of 8,000 Muslim men and boys in Srebrenica near the end of Bosnia's 1992-95 war.
The two men are on trial separately, but both deny charges of war crimes and crimes against humanity.
The former strongman Mladic, now 71, looked frail as he told the judges why he refused to testify.
"I do not recognize your court," he told presiding judge O-Gon Kwon. "It is a NATO creation. It is a satanic court."
The one-time political allies exchanged brief pleasantries before Mladic was led out of the courtroom.
"Thanks a lot, Radovan.I'm sorry, these idiots wouldn't let me speak. They defend NATO," he said as he passed, referring to the court. As he came down from the witness stand, Mladic smiled at the public gallery, which is separated from the high-security courtroom by a pane of bullet-proof glass.
Reuters contributed to this report.