Almost daily protests have gripped Serbia since November, following the collapse of a railway station roof that killed 16 people.
The tragedy became a symbol of deep-rooted corruption in the Balkan nation, with demands for a transparent investigation growing into calls for early elections.
At their peak, the protests drew hundreds of thousands on to the streets.
But the mostly peaceful demonstrations deteriorated earlier this week when large groups of pro-Government supporters – many masked and some armed with batons and fireworks – attacked protesters.
That has sparked violent clashes for several nights, leaving many injured on both sides.
Protests were further fuelled after several videos shared online showed police striking unarmed demonstrators with batons.
A video from Thursday night (local time) appeared to show around 20 officers surrounding and beating a young man in Valjevo while he lay on the ground.
Police have denied allegations of brutality, accusing demonstrators of attacking officers.
While the protests have so far led to the resignation of the Prime Minister and the collapse of the Government, Vucic has remained defiant.
He has repeatedly rejected calls for early elections and denounced the demonstrations as part of a foreign plot to overthrow him.
– Agence France-Presse