Belgrade where Karadzic reportedly was being held. Karadzic's brother, Luka, also arrived at the location in central Belgrade.
Serbian police deployed throughout central Belgrade as well as in front of the US embassy, which was targeted in nationalist rioting over Kosovo's declaration of independence in February.
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."
The former Bosnian Serb leader has topped the tribunal's most-wanted list since his indictment in July 1995 and Serbia has been under increasing international pressure to find and turn him over.
"He was at large because the Yugoslav army was protecting him. But this guy in my view was worse than Milosevic," Richard Holbrooke, former US ambassador who negotiated an end to the Bosnian War, told CNN. "He was the intellectual leader."
Holbrooke calculated the Karadzic is responsible, directly or indirectly, for the deaths of 300,000 people, because without him there would have been no war or genocide.
The charges against him, last amended in May 2000, are genocide, extermination, murder, wilful killing, deportation, inhumane acts, and other crimes committed against Bosnian Muslim, Bosnian Croat and other non-Serb civilians in Bosnia and