“His arrest gives a strong signal to the world that anyone accused of the worst crimes can be brought to justice,” said Serge Brammertz, the prosecutor for the United Nations-based war crimes tribunal in The Hague. He said international pressure to block Serbia’s entry into the European Union was a vital prod that had precipitated the arrest. According to B92, the independent Serbian broadcasting company, residents in Lazarevo said that they were unaware that Mr. Mladic was living among them, but had spotted the police early Thursday at a house reportedly belonging to Mr. Mladic’s relatives. Serbian analysts said Lazarevo had had a large population of Bosnian Serbs since World War II, some of whom would have been sympathetic to Mr. Mladic. They said he had lived in the village for two months.

“Extradition is happening,” President Tadic said, referring to The Hague. “This is the end of the search for Mladic. It’s not the end of the search for all those who helped Mladic and others to hide and whether people from the government were involved.”

Early on Thursday evening, Mr. Mladic appeared in a court in Belgrade, where a judge must decide whether all conditions have been met for Serbia to surrender him to the tribunal. But Mr. Mladic’s lawyer, Milos Saljic, said the court halted its questioning of Mr. Mladic because of his poor health. Prosecutors said the court would continue to question Mr. Mladic on Friday and that he had three days to appeal an adverse ruling by the judge.

Radovan Karadzic, the Bosnian Serb political leader and Mr. Mladic’s former boss, is being tried in The Hague on charges of genocide for his role in the Balkan bloodshed. Slobodan Milosevic, the nationalist former president of Serbia and the architect of the war, died in 2006 while his trial was under way.

Mr. Tadic, considered strongly pro-Western in the Serbian context, stressed that the arrest of Mr. Mladic “is happening on the day Catherine Ashton is coming to Serbia,” referring to the European Union’s foreign policy chief. But it was not immediately clear how the Serbian public, which has been suspicious of the West’s demands for trials of Serbs in the Balkan wars of the 1990s, would react to news of the arrest.