Tens of thousands of Serbs protest against President Aleksandar Vucic

His tight grip on power has been challenged by weeks of street protests led by university students and the rally at Belgrade's Slavija Square was one of the largest in recent years. It was part of a wider movement demanding accountability over the Nov. 1 collapse of a canopy at a railway station in the country's north that killed 15 people.

Smaller rallies were also held in the cities of Nis and Kragujevac. The rally in Belgrade started with a 15-minute silence for the victims, and later chants of “You have blood on your hands!” were heard.

Many in Serbia blame the collapse on widespread corruption and sloppy work on the railway station building in the city of Novi Sad that was twice renovated in recent years as part of questionable mega projects involving Chinese state companies. Protesters demand that Vucic and those responsible face justice.

Serbia's popular theatre and movie actors joined the protest, with actor Bane Trifunovic describing Sunday’s rally as “a festival of freedom.”

In a show of confidence, the Serbian president on Sunday inaugurated a section of a newly built highway in central Serbia. Vucic said he wouldn't budge on opposition demands for a transitional government and accused his opponents of using students to try to seize power.

“We will beat them again,” said Vucic.“They (the opposition) don't know what to do but to use someone's children.”

Prosecutors have arrested 13 people over the Novi Sad tragedy, including a government minister whose release later fuelled public scepticism about the honesty of the investigation.

The weeks long protests reflect wider discontent with Vucic's rule. The populist leader formally says he wants to take Serbia into the European Union but has faced accusations of curbing democratic freedoms rather than advancing them.

Opposition parties have said a transitional government that would prepare a free and fair election could be a way out of the political tensions as ruling populists also have been accused of rigging past votes.

Serbia’s government has extended school winter holidays by starting them nearly a week earlier to grapple with widening student protests.

Classes at universities throughout the Balkan country have been suspended for weeks with students camping inside their faculty buildings. In recent days, more high school students have joined the movement. Occasional violence has erupted when pro-government thugs tried to disrupt the protests.

A group of farmers said on Sunday that police took away the tractor which they drove into central Belgrade ahead of the protest. In addition to the farmers, Serbia’s students also have received nationwide support from all walks of life including their professors, media personalities, lawyers and prominent individuals.

Vucic initially accused the students of launching protests for money but later said he has fulfilled their demands, including publishing documentation relating to the renovation work at the Novi Sad station.