BELGRADE, Serbia — Radovan Karadzic’s defense team has filed an appeal to stop Serbia from extraditing the former Bosnian Serb leader to the U.N. war crimes tribunal, Mr. Karadzic’s brother said yesterday.

It was the first confirmation of the filing. Once Serb judges decide on the appeal — which they are likely to reject — the case will be handed over to the Serbian government, which issues the final extradition order.

Luka Karadzic visited his brother in a detention cell yesterday in the company of Mr. Karadzic’s lawyer. Asked later by reporters whether they had filed an appeal, Luka Karadzic said, “Of course we have.”

“Radovan is well, totally calm, and relaxed,” Luka Karadzic said after the visit.

Mr. Karadzic faces 11 charges at the U.N. war crimes tribunal, including genocide and conspiracy to commit genocide. He is accused of allegedly masterminding the 1995 slaughter of 8,000 Muslim men and boys in Srebrenica and the 3 1/2 year siege of Sarajevo, which left 10,000 people dead.

Officials say the war crimes suspect was captured last Monday in Belgrade, where he lived under an assumed identity. His lawyer claims that Mr. Karadzic was kidnapped July 18 and held for three days by unknown captors.

The Blic daily reported yesterday that an anonymous caller had tipped off Serbia’s security service on Mr. Karadzic’s whereabouts by saying he recognized his voice, and gave a detailed description of Mr. Karadzic’s undercover disguise and work in alternative medicine.

Mr. Karadzic was put under surveillance for six weeks before he was arrested, Blic reported. The paper is known for its well-sourced reports.

Serbia’s new, pro-Western government hopes that Mr. Karadzic’s arrest will boost the country’s bid for European Union membership. Prime Minister Cvetkovic said yesterday that the arrest proves the government’s “determination to respect international law.”

“It is no longer a question of political will,” Mr. Cvetkovic was quoted as saying by the state news agency Tanjug.

Mr. Karadzic had been a fugitive for nearly 13 years and Serbia had been accused of not searching for those sought by the U.N. tribunal.

“We don’t know where the other fugitives are hiding,” Mr. Cvetkovic said in a reference to Mr. Karadzic’s colleague and the former Bosnian Serb general, Ratko Mladic. “If we knew it, we would do as we have with Karadzic.”

Several hundred ultranationalists marched yesterday for the sixth consecutive day in downtown Belgrade in support of Mr. Karadzic, but unlike some previous days, yesterday’s rally was not violent.