When tens of thousands of protesters blocked three key bridges across the Danube River, paralyzing Serbia’s second-biggest city this weekend, the Balkan country’s beleaguered governing party issued a stern warning — not to the protesters but to the state-controlled broadcasting service for reporting on them.

After mostly ignoring three months of student-led street demonstrations across the country, Radio Television Serbia, long a propaganda bullhorn for President Aleksandar Vucic, had suddenly shifted gears and put protests in Novi Sad atop its news bulletins.

Worse still, at least for the governing party, it reported factually without denouncing the protesters as traitors in the pay of foreign intelligence services or puppets of the opposition, as it has in the past.

President Vucic’s Serbian Progressive Party complained in an unusual statement late Saturday about the “scandalous reporting” by the broadcaster, saying it “grossly abused the journalistic profession by siding with politicians who would destroy the constitutional order of the Republic of Serbia.”

Control of the media has been a central pillar of Serbia’s system under Mr. Vucic, allowing him to weather multiple rounds of protests by demonizing and discrediting protesters, and to keep a firm grip on power for more than 12 years.