Thousands of anti-government protesters marched again in Serbia on Monday with baton-wielding riot police chasing them in downtown Belgrade, prompting Serbia's populist President Aleksandar Vucic to reiterate his pledge of a planned major crackdown against the demonstrators.

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.

Serbian anti-government protesters accused police Tuesday of beating and threatening detained activists, fuelling fears of a spiralling crackdown after a week of violent clashes.