Serbian police have fired tear gas at anti-government protesters in downtown Belgrade as they clashed for the third day on the streets of the country's capital and other cities amid reports of police brutality and excessive use of force during the unrest.

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.