Radovan Karadzic is one of the world's most wanted men
Bosnian Serb war crimes suspect Radovan Karadzic, one of the world's most wanted men, has been arrested in Serbia after more than a decade on the run.
The Bosnian Serb wartime political leader disappeared in 1996.
He has been indicted by the UN tribunal for war crimes and genocide over the 1995 massacre at Srebrenica.
The appointment of a new, pro-European government in Belgrade last month appears to have cleared the way for his arrest, says a BBC correspondent.
The European Union, which the new government hopes to join, has put Serbia under considerable pressure to hand over indicted war criminals to the UN tribunal in The Hague.
But Mr Karadzic's wartime military leader, Ratko Mladic, remains at large.
'Located and arrested'
The arrest of Radovan Karadzic was welcomed by war crimes prosecutors in The Hague as a "milestone".
He has been brought before Belgrade's war crimes court, a legal procedure that indicates he may soon be extradited.
But it is not clear how soon he might be transferred to stand trial at the war crimes tribunal in The Hague, says the BBC's Bridget Kendall.
Serbian officials have suggested he will stay put for at least three days while his lawyer appeals against his extradition.
Officials said no further information about his detention would be released until the action team of prosecutors, police and intelligence teams meet in Belgrade on Tuesday morning, the BBC's Eastern Europe correspondent Nick Thorpe says.
"Radovan Karadzic was located and arrested tonight [Monday evening]" by Serbian security officers, a statement by the office of President Boris Tadic said, without giving details.
"Karadzic was brought to the investigative judge of the War Crimes Court in Belgrade, in accordance with the law on co-operation with the International Criminal Tribunal for the former Yugoslavia [ICTY]."
Serbian government sources told Reuters news agency he had been under surveillance for several weeks, following a tip-off from a foreign intelligence service.
But his lawyer, Svetozar Vujacic, said Mr Karadzic had been detained "on Friday in a bus" and held till he was brought before the judge of Serbia's war crimes court for questioning. Mr Karadzic was said to have remained silent during questioning.
Heavily armed special forces were deployed around the war crimes court in Belgrade - apparently fearing a backlash from nationalists who consider Mr Karadzic a hero.
"He did not surrender, that is not his style," his brother, Luka Karadzic, said outside the court.
'Milestone in co-operation'
Serge Brammertz, chief prosecutor of the ICTY, welcomed the arrest.
"This is a very important day for the victims who have waited for this arrest for over a decade," he said in a statement in The Hague.
Eleven counts of genocide, war crimes, crimes against humanity and other atrocities
Charged for the killing of some 12,000 civilians during the siege of Sarajevo
Allegedly organised the massacre of at least 7,500 Muslim men and youths in Srebrenica
Targeted Bosnian Muslim and Croat political leaders, intellectuals and professionals
Unlawfully deported and transferred civilians because of national or religious identity
Destroyed homes, businesses and sacred sites
In the Bosnian capital, Sarajevo, crowds spilt onto the streets to celebrate news of the arrest as cars streamed through the streets sounding their horns.
"This is the best thing that could ever happen, you see people celebrating everywhere. I called and woke up my whole family," Sarajevo resident Fadil Bico told Reuters.
Richard Holbrooke, the US diplomat who brokered the Dayton Peace Accord for Bosnia in 1995, told the BBC that "a major, major thug has been removed from the public scene".
"One of the worst men in the world, the Osama Bin Laden of Europe, has finally been captured," Mr Holbrooke told BBC World News America.
Ljiljana Zelen-Karadzic, wife of Radovan Karadzic, said her daughter had called her to break news of the arrest to her.
"As the phone rang, I knew something was wrong. I'm shocked, confused. At least now, we know he is alive," she told the Associated Press.
The arrest of Mr Karadzic and other indicted war criminals is one of the main conditions of Serbian progress towards European Union membership.
French Foreign Minister Bernard Kouchner, whose country holds the EU's rotating presidency, said a major obstacle to Serbian membership had been lifted.
Srebrenica was the scene of the worst massacre in the Bosnian war
Mr Karadzic denied the charges against him soon after the first indictment and refused to recognise the legitimacy of the UN tribunal.
The UN says Mr Karadzic's forces killed at least 7,500 Muslim men and boys from Srebrenica in July 1995 as part of a campaign to "terrorise and demoralise the Bosnian Muslim and Bosnian Croat population".
He was also charged over the shelling of Sarajevo, and the use of 284 UN peacekeepers as human shields in May and June 1995.
After the Dayton accord that ended the Bosnian war in 1995, the former nationalist president went into hiding.
International pressure to catch Mr Karadzic mounted in spring 2005 when several of his former generals surrendered and a video of Bosnian Serb soldiers shooting captives from Srebrenica shocked television viewers in former Yugoslavia.
He had been a close ally of former Yugoslav President Slobodan Milosevic, who was himself extradited to The Hague tribunal in 2001, but died in 2006, shortly before a verdict was due to be delivered in his case.