By Ivana Sekularac

KRAGUJEVAC, Serbia (Reuters) - Tens of thousands of students descended upon the central Serbian city of Kragujevac on Saturday in the biggest show of anger yet against the government over a deadly railway disaster in November.

In the three months since 15 people were killed when a roof collapsed at a newly-renovated train station in the city of Novi Sad, mass demonstrations have grown into Serbia's biggest protest movement in years and the starkest threat to populist President Aleksandar Vucic's decade-long grip on power.

On Saturday morning in near-freezing temperatures, students arrived in Kragujevac from across Serbia, by bus and even on foot from the capital Belgrade some 140 km away, beating drums, blowing whistles and holding up the country's flag. Locals greeted them with smiles and cheers.

The protesters planned to blockade one of the city's main boulevards for 15 hours and 15 minutes, in a symbolic tribute to the victims. They held 15 minutes of silence.

"We are here to fight for a better tomorrow, against corruption," said 20-year-old Jovan, who drove from Belgrade with a group of friends.

By noon, all roads leading to Kragujevac were jammed with traffic.

Joined by teachers, farmers and other workers, the students have enjoyed support from the wider public, with many Serbians blaming the Novi Sad tragedy on corruption and nepotism within Vucic's government, allegations authorities deny.

As the numbers swelled in Kragujevac, locals brought the protesters homemade pies and hot tea to keep them going.

Verica, a 52-year-old professor from the city, joined the protest with her teenage daughter.

"I have not been so happy and proud for a long time," she said. "I am proud of their persistence."

As public dissent grows, Vucic was expected to deliver a speech to his supporters in the northwestern town of Sremska Mitrovica on Saturday evening.

STUDENT DEMANDS

Prime Minister Milos Vucevic and two other ministers have resigned over the protests and prosecutors have charged 13 people over the disaster but this has done little to quell the demonstrations.

Since the accident, students have staged daily protests, taking over university buildings and blocking highways and squares.

"We will stay here until the blockade ends," said Djordje Vujovic, 22, a mechanical engineering student.

"We want to be able to continue our studies but only when our demands are met," he said.

The students are demanding that authorities publish documents relating to the station roof collapse; justice for those responsible; the dismissal of charges against protesting students; and a greater budget for higher education.

Ivan and Ivana, 23-year-old mathematics students, were among a group of around 400 people who had walked for four days to reach Kragujevac from Belgrade.

"We wanted to show the people living in the country that we support them," Ivan said. "We do not think only about the people in Belgrade."

(Additional reporting by Aleksandar Vasovic in Belgrade; Writing by Karolina Tagaris; Editing by Kirsten Donovan)