Karadzic was indicted along with his army commander, General Ratko Mladic, for genocide at Srebrenica, where some 8,000 unarmed Bosnian Muslim males were rounded up and murdered and bulldozed into mass graves in July 1995.
"The arrest of Radovan Karadzic is confirmation that every criminal will eventually face justice," said Munira Subasic, head of a Srebrenica widow's association.
"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 beloved ones," she said.
The arrest came on the eve of a meeting of EU foreign ministers scheduled to discuss closer relations with Belgrade following the formation of a new pro-Western government.
"It proves the determination of the new Serbian government to achieve full cooperation with the ICTY (International Criminal Tribunal on the former Yugoslavia). It is also very important for Serbia's European aspirations," said European Commission President Jose Manuel Barroso.
EU members who insist Serbia must hand over all war crimes suspects are likely to see it as proof that Karadzic's fellow genocide suspect, the fugitive wartime commander Mladic can also be seized if Belgrade has the political will to face down hardline nationalists.
The new government is an odd-couple alliance of President Tadic's pro-Western Democratic Party and the Socialists of the late Slobodan Milosevic, who died in detention at the Hague war crimes prison before del Ponte could obtain a conviction.
UN WELCOMES ARREST
U.N. Secretary-General Ban Ki-moon praised Serbian authorities for taking a "decisive step toward ending impunity" of those indicted for crimes in Balkan conflicts.
"This is a historic moment for the victims, who have waited 13 years for Mr. Karadzic to be brought to justice," he said in a statement released by the United Nations.
The EU, initially worried about where the Socialists stood on the Hague tribunal, welcomed the capture, which apparently took place in Belgrade.
Some officials said they expected Karadzic to be taken swiftly to The Hague, but a senior judicial source in Serbia told Reuters he would likely stay in the country for at least three days while his lawyers appeal to fight his extradition.
Karadzic is still seen by militant nationalists as a national saviour following the collapse of Yugoslavia.
"This is payback to the EU for bringing this new government to power," said Aleksandar Vucic of the nationalist Radicals, one of Serbia's strongest parties. "Karadzic is a Serbian hero. There will be a strong backlash."
Karadzic's place of hiding had been a constant subject of international speculation since he went underground in 1997. The West had long suspected Belgrade of failing to press the search, but the new government had signalled it wanted to comply.
