Karadzic, one of the world's top war crimes fugitives, was accused of masterminding mass killings that the tribunal described as "scenes from hell, written on the darkest pages of human history."

Charged with organizing the 1995 massacre of 8,000 Muslims in Srebrenica among other atrocities of the Bosnian war, Karadzic topped the tribunal's most-wanted list for more than a decade, allegedly resorting to elaborate disguises to elude authorities.

"This is a very important day for the victims who have waited for this arrest for over a decade," the tribunal's head prosecutor, Serge Brammertz, said. "It clearly demonstrates that nobody is beyond the reach of the law and that sooner or later all fugitives will be brought to justice."

A Serbian police source, speaking on condition of anonymity because he was not allowed to speak to the media, said Karadzic was arrested in a Belgrade suburb after weeks of surveillance of his safe house and a tip from a foreign intelligence service.

President Boris Tadic's office said Karadzic was arrested "in an action by the Serbian security services" and taken before the investigative judge of Serbia's war crimes court, a legal procedure that indicated he would soon be extradited to the UN war crimes court in The Hague, Netherlands.

Investigative judge Milan Dilparic said early Tuesday that Karadzic "is now being questioned" - the first step in an extradition process that includes presenting Karadzic with the indictment, and a three-day period for him to appeal any decision to hand him to the Hague court.

Heavily armed special forces of the Serbian Gendarmerie were deployed around the war-crimes court in Belgrade - apparently fearing a backlash from nationalists who consider Karadzic their war hero.

"He did not surrender, that is not his style," his brother Luka Karadzic said outside the court.

Dozens of Karadzic supporters, chanting "Karadzic Hero!" and "Tadic Traitor!" gathered near the building and several were arrested after attacking reporters in front of the courthouse.

Other officers took up positions throughout central Belgrade and in front of the US embassy, which was targeted in nationalist rioting over Kosovo's declaration of independence in February.

Serbia has been under heavy pressure from the European Union to turn over suspects to the international tribunal, which indicted Karadzic on genocide charges in 1995.

Still, his arrest came as a surprise to many. He whereabouts had been a mystery to UN war crimes prosecutors unlike those of his wartime military commander, Gen. Ratko Mladic, who had last been spotted living in Belgrade in 2005 and remains at large.

The nationalists lost power in Serbia when a new pro-Western government took over last month, removing the nationalist official who held the office of secret police chief, the official formally in charge of arresting war crimes suspects.

"It is clear that those changes led to Karadzic's arrest," prominent Serbian human rights activist Natasa Kandic said. She said that while on the run Karadzic had been cared for several times in Belgrade's military hospital, which she called "a haven for Serbian war criminals."

In Sarajevo, downtown streets were jammed with honking cars and euphoric crowds as Bosnian Muslims celebrated Karadzic's arrest.

Richard Holbrooke, the US diplomat who brokered the Dayton deal under President Clinton, said Karadzic was responsible for the deaths of 300,000 people and his arrest marked "a historic day."

"A man who has been on the run for 12 years, who NATO should have captured, has been captured and by the Serb government themselves," Holbrooke told CNN. "This guy was a kind of a Robin Hood to the Bosnian Serbs, evading capture for 12 years, fomenting dissent. His removal from the scene will help enormously to create stability."

The White House called the arrest "an important demonstration of the Serbian government's determination to honor its commitment to cooperate with the International Criminal Tribunal for the former Yugoslavia."

On July 11 tens of thousands of people commemorated the 13th anniversary of the Srebrenica massacre, which saw Serb troops led by Mladic overrun an enclave supposedly protected by outnumbered UN troops who never fired a shot.

Mladic's troops rounded up the entire population of Srebrenica and took the men away for execution.
"There is no better tribute to the victims of the war's atrocities than bringing their perpetrators to justice," the White House said.

If Karadzic is extradited to the tribunal in The Hague, he would be the 44th Serb suspect sent there. The others include former President Slobodan Milosevic, who was ousted in 2000 and died in 2006 while on trial on war crimes charges.

A statement from the EU presidency, currently held by France, said the arrest "illustrates the commitment of the new Belgrade government to contributing to peace and stability in the Balkans region" describing it as "an important step on the path to the rapprochement of Serbia with the European Union."

"This news gives us immense satisfaction. The new government in Belgrade stands for a new Serbia, for a new quality of relations with the EU," EU foreign affairs chief Javier Solana said. "Radovan Karadzic will be facing a tribunal, having a fair trial, responding for many crimes. This is a good day for justice in the Balkans."

As leader of Bosnia's Serbs, Karadzic hobnobbed with international negotiators and his interviews were top news items during the 3½-year Bosnian war, set off when a government dominated by Slavic Muslims and Croats declared independence from Yugoslavia in 1992.

His indictment alleges that he, acting together with others, committed the crimes to secure control of areas of Bosnia which had been proclaimed part of the "Serbian Republic" and significantly reducing its non-Serb population.

His life changed by the time the war ended in late 1995 with an estimated 250,000 people dead and another 1.8 million driven from their homes. He was indicted twice by the UN tribunal on genocide charges stemming from his alleged crimes against Bosnia's Muslims and Croats.

Karadzic's reported hide-outs included Serbian Orthodox monasteries and refurbished mountain caves in remote eastern Bosnia. Some newspaper reports said he had at times disguised himself as a priest by shaving off his trademark silver mane and donning a brown cassock. Others said he wore women's wigs.

His wife, Ljiljana, told The Associated Press by phone from her home in Karadzic's former stronghold, Pale, near Sarajevo that her daughter Sonja had called her before midnight.

"As the phone rang, I knew something was wrong. I'm shocked. Confused. At least now, we know he is alive," Ljiljana Karadzic said, declining further comment.

Under the UN indictment, last amended in May 2000, the UN war crimes tribunal charged Karadzic with 15 counts of genocide, war crimes, crimes against humanity and other atrocities committed between 1992 to 1996.

Those counts include six counts of genocide and complicity in genocide; two counts of crimes against humanity and two counts of deportations and other inhumane acts; and one count each of persecution, inflicting terror on civilians, taking hostages, violating laws of war and gravely breaching the Geneva Conventions.