(ANSA-AFP) - BELGRADE, JUN 28 - Serbian protesters are expected to gather in their tens of thousands in the capital Belgrade on Saturday, issuing an "ultimatum" for the government to call early elections after months of student-led strikes. Anti-graft protests have rocked the Balkan nation since November when a train station roof collapse in the northern city of Novi Sad killed 16 people -- a tragedy widely blamed on entrenched corruption. For more than half a year, students have blockaded universities and organised large demonstrations around the country, demanding a transparent investigation into the deaths. With little action from authorities, their focus shifted last month to calls for early parliamentary elections. Ahead of Saturday's protest, students issued the "ultimatum" to President Aleksandar Vucic, with a deadline of 9 pm (1900 GMT), three hours after the protest is set to begin in one of Belgrade's main squares. Vucic responded on Friday, again rejecting the student's demands for immediate elections, having previously stated that a national poll would not be held before the end of 2026. "The ultimatum was not accepted, you don't have to wait until 9 pm tomorrow," he told state television station RTS. (ANSA-AFP).