BELGRADE, Serbia — Former Bosnian Serb President Radovan Karadzic, accused architect of massacres and the politician considered most responsible for the deadly siege of Sarajevo, was arrested Monday evening in a Serbian police raid ending his 13 years as the world's most-wanted war crimes fugitive.

His alleged partner in the persecution and "cleansing" of tens of thousands of Bosnian Muslims and Croats, former Bosnian Serb military chief Ratko Mladic, remained at large.

A psychiatrist turned diehard Serbian nationalist politician, Karadzic is the suspected mastermind of mass killings that the U.N. war crimes tribunal described as "scenes from hell, written on the darkest pages of human history." They include the 1995 massacre of 8,000 Muslims in Srebrenica, Europe's worst slaughter since World War II.

"This is a very important day for the victims who have waited for this arrest for over a decade. It is also an important day for international justice because it clearly demonstrates that nobody is beyond the reach of the law," said Serge Brammertz, the tribunal's head prosecutor.

A Serbian police source said Karadzic was arrested in a Belgrade suburb after weeks of surveillance of his safe house and a tip from a foreign intelligence service.

Sveta Vujacic, Karadzic's lawyer, said the fugitive had been arrested on a public bus Friday and held until he was brought to the court Monday.

"He just said that these people showed him a police badge and than he was taken to some place and kept in the room. And that is absolutely against the law what they did," Vujacic said.

Could be sent to U.N. court

Karadzic, who turned 63 Saturday, was taken before the investigative judge of Serbia's war crimes court, Serbian President Boris Tadic's office said. The move indicates he could soon be extradited to the U.N. court at The Hague, Netherlands, but authorities have not confirmed plans to send him there. He could also be tried locally. Investigative judge Milan Dilparic said early today that Karadzic was "being questioned."

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

Serbia braced for a possible reaction from ultra-nationalists who are believed to have helped shelter Karadzic and Mladic over the years.

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."

In the Bosnian capital of Sarajevo — besieged throughout the war by Bosnian Serb nationalists — streets were jammed late Monday as Bosnian Muslims celebrated the arrest.

Operating from their stronghold in Pale above the city, the Serbs starved, sniped and bombarded the center of Sarajevo, controlling nearly all roads. Inhabitants were kept alive only by food aid and supplies provided by U.N. donors and peacekeepers.

Serbia has been under increasing international pressure to find Karadzic and turn him over.

The charges against him, last amended in May 2000, include genocide, extermination, murder, deportation, inhumane acts, and other crimes committed against Bosnian Muslim, Bosnian Croat and other non-Serb civilians in Bosnia during the 1992-95 war.

"These offenses include a brutal campaign of ethnic cleansing directed at non-Serbs, organized attacks on places of worship, the operation of concentration camps, and the mass murder of thousands of Bosnian Muslim and Bosnian Croat civilians," the White House statement added.

Karadzic was born to a poor rural family in Montenegro. He trained as a psychiatrist and moved to Sarajevo with his wife and two children in the 1960s.

Party founder, president

A flamboyant gambler and sometime poet, Karadzic became a founding member of the Serbian Democratic Party in Bosnia in 1990. Two years later, he was elected president of the three-person presidency of the Serbian republic in Bosnia, which had just been freshly recognized as an independent state by the United Nations.

He became sole president of the Serb Republic in Bosnia that year, remaining in that position until 1996 and also serving as supreme commander of the armed forces.

Karadzic hobnobbed with international negotiators and his interviews were top news items during the 3-year Bosnian war. But his life changed by the time the war ended in late 1995 with tens of thousands of dead and 1.8 million driven from their homes. He was indicted twice by the U.N. 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.