LONDON — Ratko Mladic, the former Bosnian Serb military commander facing war crimes charges in The Hague, refused to testify on Tuesday at the trial of his former political ally, Radovan Karadzic, saying he did not wish to prejudice his own case and was medically unfit to give evidence.

The appearance of the two men in the same courtroom represented an unusual reunion. Mr. Mladic and Mr. Karadzic, the former Bosnian Serb political leader, have not been seen together in public for years. They are accused in different cases of overseeing and carrying out some of the worst atrocities of the Bosnian war from 1992 to 1995, including the execution of thousands of captives at Srebrenica and the siege of Sarajevo.

In a brief and frequently disrupted hearing that veered between anger and farce, Mr. Mladic delivered a fiery condemnation of the tribunal, calling it a “satanic court,” and forced a delay in the proceedings, which the court streamed online, by saying he was not wearing his dentures and could not speak properly until they were brought to him.

Even when Mr. Karadzic asked his former military chief a series of questions — including one about the causes of the sniping and shelling by Bosnian Serb forces surrounding Sarajevo — Mr. Mladic refused to answer.