Witnesses said U.S. troops serving with the NATO peacekeeping mission entered the home of Sasa Karadzic, 32, in Pale, handcuffed him and put a flak-jacket on him before whisking him off by helicopter.
Several military vehicles and two helicopters were engaged in the operation.
Sasa Karadzic's mother Ljiljana and sister Sonja characterized the arrest as a "kidnapping" and repeated that the family had not had any contact with Radovan Karadzic for many years.
Pale, a former ski-resort in the mountains above Sarajevo in central Bosnia, was the seat of Radovan Karadzic's government during the 1992-95 Bosnian war.
A NATO spokesman, Derek Chappell, confirmed that an operation was ongoing, but refused to provide any details.
The homes of the Karadzic family - his wife Ljiljana, daughter Sonja and Sasa - were searched in May in Pale. Documents and other items were taken from them on that occasion.
As president of the self-proclaimed Bosnian Serb republic, Radovan Karadzic, now 60, was accused by The Hague-based United Nations war crimes tribunal of genocide and other war crimes targeting the Moslem population.
He has remained at large despite a massive international presence in Bosnia since 1996. He is presumed to be hiding in the Bosnian, possibly also Montenegrin, mountains with the help of an underground network. dpa zl bb ms mga ds
- Deutsche Presse Agentur
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