Several hundred protesters gathered in Serbia's capital, Belgrade, to demand free and fair elections ahead of local polls scheduled next year.
The protest on October 6 was organized by the Free Citizens movement led by former ombudsman and opposition figure Sasa Jankovic.
Protesters demanded equal treatment in the media and international observers at the Belgrade vote in April.
Opposition leaders have alleged that the ruling parties are planning election fraud in the vote.
The elections are being widely viewed as a test of President Aleksandar Vucic's rule.
Vucic secured victory in the presidential election earlier this year and his conservative coalition controls the government, but opposition parties are hoping to undermine his power in Belgrade.
Opposition figures have accused Vucic of stifling democratic freedoms and cracking down on the media.
With reporting by AP