The situation has put pressure on Serbia's increasingly autocratic President Aleksandar Vucic, who has refused the protesters' demand to call an early parliamentary election.

Violence escalated across Serbia as anti-government protests entered their fifth consecutive night. In Valjevo, masked protesters set fire to the ruling Serbian Progressive Party (SNS) offices before clashing with riot police.

Anti-corruption protesters in Serbia set fire to the Valjevo offices of the country's ruling political party, city leaders said, amid clashes sparked by the deadly collapse of a rail station in November.