Fueling fears of clashes, Vucic's supporters have been camping in central Belgrade in front of his headquarters. The crowd included ex-members of a dreaded paramilitary unit involved in the assassination in 2003 of Serbia's first democratic Prime Minister Zoran Djindjic, as well as soccer hooligans who are known for causing violence.

A deafening sound of whistles and vuvuzelas echoed throughout the Serbian capital, on high alert since the rally was announced. It's the latest in a series of anti-corruption protests that have shaken populist President Aleksandar Vucic's 13-year firm grip on power.