Baton wielding police fought running battles with small groups throwing stones and bottles and setting off firecrackers. The skirmishes took place in several locations in the center of the capital on Sunday. There were no immediate reports of injuries or arrests.
The trouble began after thousands of demonstrators, many singing nationalist songs and carrying banners honoring Mladic, poured into the street outside Serbia's parliament to demand the release of the wartime Bosnian commander, whom they consider a hero.
More than 3,000 riot police were deployed around government buildings and Western embassies, fearing that the demonstration could turn violent, as similar rallies have in the past. Riot police tried to block small groups of extremists from reaching the rally.
Dusan Stojanovic and Danica Kirka contributed.