Serbian police say nearly 50 protesters have been detained and dozens of people injured during clashes at anti-government protests in Serbia against the populist rule of President Aleksandar Vucic.

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.

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.