Radovan Karadzic is to fight Serbia's attempt to extradite him to The Netherlands to face a war crimes court.
Karadzic's lawyer vowed to appeal against the extradition of the former Bosnian Serb chief to a United Nations tribunal.
Meanwhile, Serb nationalists attacked police in the capital Belgrade, lashing out against the new Western-leaning government that captured Karadzic, 63.
Riot police deployed in downtown Belgrade yesterday to keep about 200 members of the extremist Obraz group under control, but the demonstrators threw stones and clay pots at the officers, chanted: "Treason!" and tried to break through cordons.
Five demonstrators and a policeman were injured, doctors at Belgrade emergency clinic said.
"This is a hard day for Serbia," Tomislav Nikolic, leader of the ultra-nationalist Serbian Radical Party said. "(Karadzic was) a legend of the Serbian people."
From the village of Petnjica where Karadzic was born, a disgusted relative, Vukosav Karadzic, said he was "sorry he did not kill himself, but allowed himself to be captured".
Serb officials say they arrested the former wartime leader on Monday evening near Belgrade after more than a decade on the run. Karadzic had grown a long, white beard to conceal his identity and had lived freely for months in the capital before being arrested.
"His false identity was very convincing," said Vladimir Vukcevic, Serbia's war crimes prosecutor who co-ordinated the security forces arrest. "He had moved freely in public places."
Karadzic is sought on 11 charges of genocide, war crimes and crimes against humanity for his actions during Bosnia's 1992-1995 war. The psychiatrist-turned Serbian nationalist is accused of masterminding the deadly wartime siege of Sarajevo and the 1995 executions of up to 8,000 Muslim boys and men in Srebrenica.
While on the run in Serbia, the world's top fugitive worked at a private clinic and wrote for a Belgrade magazine, according to Serbian officials. Karadzic also lectured about meditation at a May festival in Belgrade.
To do all this, Karadzic used a false name: Dragan Dabic, government minister Rasim Ljajic said.
Mr Ljajic refused to reveal more details about Karadzic's arrest, saying Karadzic's movements were being analysed to help track down another war crimes suspect still at large, Bosnian Serb wartime commander Ratko Mladic.
Serbian security services found Karadzic while looking for Mladic, he said.
Karadzic was questioned early yesterday by a Serbian judge who later ruled that he could be handed over to the UN war crimes tribunal in The Hague, Netherlands, Mr Vukcevic said. Karadzic has three days to appeal.
His lawyer, Sveta Vujacic, said he would fight extradition on the last day, Friday, to thwart authorities' wishes for Karadzic's immediate transfer.
Karadzic appeared to have been arrested because of a change in political will.
Serbian president Boris Tadic's pro-Western government came to power only two weeks ago and appointed a new security chief, replacing an aide to former nationalist prime minister Vojislav Kostunica.
Liberal politician Nenad Canak said Mr Kostunica and his nationalists had protected Karadzic and Mladic for years and cherished their nationalist ideas.
That assessment matched comments from the former US ambassador to Bosnia, Richard Holbrooke, who said: "(Karadzic) was protected by people who knew who he was.
"Nato did not make an all-out effort to capture him at the beginning, in 1996, which was a terrible mistake. They knew exactly where he was. And then he slipped away and he was sheltered by people."
There was no immediate comment from Mr Kostunica on Karadzic's arrest.
European Union foreign ministers said the arrest helped Serbia's bid for EU membership, and German chancellor Angela Merkel called the arrest a "historic moment".
"The victims need to know: massive human rights violations do not go unpunished," she said in Berlin.
Karadzic's whereabouts had been a mystery since he went on the run in 1998, with his early hideouts reportedly including monasteries and mountain caves in remote eastern Bosnia.
Karadzic's family in Bosnia, banned from leaving the country over suspicions that they helped him elude capture, have asked to have the restrictions lifted.
His daughter Sonja said family members wanted to spend at least a few hours with Karadzic before his transfer to UN custody.
"For years we have not seen our father, husband and grandfather; my mother's health is not very good, and we do not have the financial means necessary to travel to Netherlands," she said.