Hundreds of Serbian students and others briefly blocked traffic throughout the country Friday as political tensions continue to simmer following last month’s collapse of a concrete canopy that killed 15 people.

The 15-minute traffic blockades have been held every Friday at 11:52 a.m., the exact time on Nov. 1 when tons of concrete came crashing down on people standing underneath outside a railway station building in the northern city of Novi Sad.

Initially, 14 people died and three suffered injuries. Another person died later in hospital.

Many in Serbia blame the collapse on widespread corruption and sloppy renovation work on the railway station building that was twice renovated an recent years as part of railway upgrading projects with Chinese state companies.

Prosecutors have arrested 13 people including a government minister whose release later fueled public skepticism about the investigation.

Separately, Serbia’s government extended school winter holidays by starting them nearly a week earlier as its continues to grapple with widening student protests challenging the rule of populist President Aleksandar Vucic. The move is seen as a government ploy to curtail 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.

The protests reflect a broader dissatisfaction with the increasingly autocratic rule of Vučić. Serbia is formally seeking European Union entry but Vucic has faced accusations of curbing rather than advancing democratic freedoms.

Political tensions have flared up in occasional violence when pro-government thugs tried to disrupt the protests.

In the latest incident Thursday, opposition leader Dragan Djilas reportedly brawled with Serbian Progressive Party who were putting up posters against him. Government officials accused Djilas of attacking the ruling party members. Djilas said was he and his associate who were kicked and had to be treated in hospital for multiple injuries.

Serbia's students have received nation-wide support from their professors, actors, lawyers, farmers and prominent individuals. Serbia's tennis star Novak Djokovic has offered veiled backing, saying he stands by the young but without mentioning the protests or the student demands.

Students and farmers together are planning a big rally on Sunday in Belgrade.

Vucic initially accused the students of launching protests for money. He has also accused opposition parties of being "political pedophiles" and using children to achieve own political goals.