The protest, coinciding with Europe Day, was organised by three opposition parties -- the Liberal Democratic Party, the Civic Alliance and the Social Democratic Union -- and eight non-governmental organisations.
Protesters called on the government of Prime Minister Vojislav Kostunica to step down for failing to arrest fugitive Bosnian Serb wartime military commander Ratko Mladic, which prompted the EU to suspend talks on a Stabilisation and Association Agreement last week, and demanded early elections.
"Serbia is not hostage to Mladic but to the incompetent Kostunica government," one of the opposition leaders said.