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.

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.