Serbian police had detained one of three men who allegedly broke into the offices of a secular Muslim party and threatened its activists.
Witnesses said three assailants broke into the office of the Sandzak Democratic Party late Wednesday in the Belgrade suburb of Zemun traditionally a stronghold of the extreme nationalist Serbian Radical Party, shouting at party officials to leave Belgrade.
They also tore down posters of Serbia's moderate President Boris Tadic, witnesses told B92 TV.
"This is the most serious attack on our party so far," said party leader Rasim Ljajic, referring to repeated threats against non-Serbs by nationalists.
Ljajic, a moderate, has played a prominent role in promoting ethnic tolerance. He also heads the national council for cooperation with the U.N. war crimes court in The Hague, Netherlands.
That court has indicted Mladic, the former Bosnian Serb commander, with genocide for allegedly orchestrating the 1995 massacre of some 8,000 Muslims in Srebrenica. He remains at large and is considered a hero by many nationalists in Serbia.
Ljajic's party is based in Sandzak, a predominantly Muslim region in southwestern Serbia, but is also active in Belgrade and has close ties to local pro-Western parties.
The party said the attackers called the activists in the Zemun office "Turks" a characterization used by Serb nationalists as a derogatory term for Muslims and praised Mladic's role in the 1992-95 Bosnian war.
Serbia has recently seen a rise in nationalism, likely stemming from Serbs' frustration over Western plans to grant independence to Kosovo.