The offices of the ruling Serbian Progressive Party (SNS) have been set on fire during a fifth night of unrest in a row that saw fresh clashes between anti-government protesters and riot police.

Police in the city of Valjevo reportedly used stun grenades and tear gas on protesters after a small group of masked individuals attacked the empty facilities of the SNS.

There were widespread allegations of violence and police brutality in the capital, Belgrade, and Novi Sad. Serbia’s interior ministry has denied these allegations.

It comes as Russia pledged to assist the beleaguered pro-Moscow President Aleksandar Vučić, who leads the SNS, saying it would not “remain unresponsive”.

The protests were initially triggered by a railway station collapse in Novi Sad in November last year that killed 16 people, with many blaming corruption and corner-cutting by Vučić’s government for the disaster.

While anti-corruption demonstrations have drawn in hundreds of thousands of protestors, they had been largely peaceful until Wednesday’s clash, when pro-government loyalists staged counter-demonstrations.

On Saturday night, riot police were again deployed in a number of cities including Belgrade as people took part in demonstrations to demand early elections.

Offices and flags representing Vučić’s SNS party had been a focus of the protesters’ anger.

Protesters also smashed the windows of the headquarters of the Serbian Radical Party, a coalition partner of the SNS.

In the past week, injuries have been reported at protests across the country and unverified video has circulated on social media of police beating a man in Valjevo.

Michael O’Flaherty, the Council of Europe’s commissioner for human rights, raised concern on Friday over the “police’s disproportionate force” in Serbia, urging authorities to “end arbitrary arrests and de-escalate the situation”.

Vučić responded to Saturday’s developments on Instagram, writing that “violence is an expression of total weakness” and promising to “punish the bullies”.

He has repeatedly rejected calls for early elections and denounced the demonstrations as part of a foreign plot to overthrow him.

Russia’s Foreign Ministry has offered its support to the pro-Moscow, right-wing president.

It said in a statement that “we cannot remain unresponsive to what is happening in brotherly Serbia”.

The statement said that police were “using lawful methods and means to contain the violent mobs” and that “public order, security, and human lives” were at risk.

Protests have gripped Serbia almost daily since the Novi Sad railway station collapse.

The tragedy became a symbol of entrenched corruption in the Balkan nation, with initial calls for transparent probes growing into demands for early elections and the end to Vučić’s 12-year reign.

At their peak, the protests drew hundreds of thousands on to the streets.