Tens of thousands of people flocked to the southern town of Nis on Saturday to commemorate victims of a railway station disaster and take part in a student-led protest that has become the biggest threat yet to the Serbian president’s grip on power.

Mass demonstrations have gathered momentum in the four months since 15 people were killed when a roof collapsed at a newly renovated train station in Novi Sad, Serbia’s second-biggest city.

Many Serbians blame the collapse on corruption they link to President Aleksandar Vucic’s decade in power, and teachers, farmers and other workers have joined the protests that began with a student blockade of classes at universities in December.

Vucic’s government has said it will launch an anti-corruption campaign, and has denied allegations of corruption.

At 11:52 a.m., the time when the roof collapsed on November 1, thousands stood in silence in the central streets of Nis to commemorate the victims.

“This is the way to make a revolution,” said Tarek, a 22-year-old graphic designer from the city of Novi Pazar. “It is the right way to achieve the change, to create a better future, a functional state without corruption.”

Hundreds of students marched to Nis for the protest and were joined by about 1,500 people on motor bikes.

Among the protesters were Anja and Ana, 20-year-old students who walked to Nis from the town of Bor about 130 km (80 miles) away.

“Walking to Nis, I think, we managed to wake up Serbia. We feel good because of that and we could walk back another 130 kilometres,” said Anja, who did not give her full name.

“This is not the society we would like to live in. We want a better future.

Prime Minister Milos Vucevic and two other ministers have resigned over the protests, which have taken place in the capital Belgrade and in towns across the country.

Prosecutors have charged 13 people in connection with the roof collapse.

But the students have continued daily protests, pressing their demands that authorities publish documents relating to the station roof collapse and justice for those responsible. They also want charges against the protesting students to be dismissed, and an increased budget for higher education.

To boost support, Vucic toured eastern Serbia, meeting farmers and citizens. “There will be no colour revolution,” Vucic said in the eastern town of Majdanpek on Saturday.

On Friday, deputy prime minister Aleksandar Vulin accused Western intelligence services of attempting to stage a “colour revolution” in Serbia - a reference to popular uprisings experienced by former Soviet states.