EU foreign ministers were due to discuss closer ties with Serbia's new, pro-Western government. U.N. Secretary-General Ban Ki-moon praised Belgrade for taking a "decisive step toward ending impunity" of war crime suspects in the Balkan wars.
Karadzic was indicted in 1995 along with his army commander, General Ratko Mladic, for genocide in Sarajevo and Srebrenica, where unarmed Bosnian Muslim males were rounded up, murdered and bulldozed into mass graves.
Richard Holbrooke, U.S. envoy during the wars of the 1990s' described Karadzic as "a real, true architect of mass murder".
Munira Subasic, head of a Srebrenica widow's association said the arrest "is confirmation that every criminal will eventually face justice."
"I hope that people who had to keep quiet because of Karadzic will start revealing the locations of mass graves and let us find the truth about our loved ones," she said.
Karadzic went underground in 1997 to evade the huge force of NATO peacekeepers that deployed in Bosnia at the end of the war, with part of their brief to find and arrest him.
Alleged sightings were rare. He was said to be hiding in monasteries, disguised, moving between remote hideouts with the help of a network of diehard loyalists.
HUNT FOR MLADIC
His arrest leaves Mladic and Croatian Serb suspect Goran Hadzic still on the run. Serb officials have refused to give exact details on the operation to arrest Karadzic, saying they did not want to blow the chances of arresting Mladic and Hadzic.
"I appeal to the rest of the Hague indictees to surrender," said Serbian Defence Minister Dragan Sutanovac.
Karadzic's arrest showed the two-week-old Serbian government putting pragmatism over pride to help push Serbs towards the EU. The government groups pro-Western Democrats, and Socialists once led by Milosevic, who died in detention at The Hague in 2006.
Many Serbs see the tribunal as biased and prone to laying all the blame for the wars in Croatia, Bosnia and Kosovo on Serbia, but most are keenly aware of the practical benefits of EU membership for their impoverished country.
Nationalists who see Karadzic and Mladic as defenders of the Serb nation staged a few low-key protests.
"This is a dark day in Serbian history. Radovan Karadzic is not a war criminal. He has become a legend," said Tomislav Nikolic of the nationalist Radicals.
