Hadzic was arrested Wednesday morning near a village north of Belgrade. He is wanted by the International Tribunal for the Former Yugoslavia in The Hague, Netherlands, on charges of war crimes committed against Croats and other non-Serbs in eastern Slovenia from 1991-93.
NATO Secretary-General Anders Fogh Rasmussen welcomed the arrest as a sign of Serbia's willingness to live up to its international obligations.
"Following the transfer of (former Bosnian Serb commander) Ratko Mladic to the Hague (in May), this arrest will allow for the most painful chapter in recent European history to be closed," said Rasmussen in a statement.
The European Union's enlargement commission said in February that Serbia was making progress on membership requirements but it was concerned about issues related to fugitives from the Yugoslav wars in the 1990s.
Italian Foreign Minister Franco Frattini said from China that the arrest of Hadzic meant there weren't any obstacles to Serbia's entrance into the EU.
"Serbia's door to the EU has been opened wide," he said in a statement.