New Delhi, Oct. 27 -- The reason: the park in Belgrade had been cordoned off and occupied by throngs of Vucic's most ardent - and violent - supporters, an effective enforcer squad for the authoritarian leader tasked with intimidating and sometimes assaulting members of a growing anti-government student movement.

But after Vucic stunned the Balkan country Sunday evening by conceding defeat in an election against student-backed candidates, ecstatic young revelers filled the streets of Belgrade and ripped down the cordons around the encampment known as Caciland. Before long, the barricades lay tossed aside and the tents within went up in smoke, along with Vucic's power.

The seismic shift came after two years of relentless organizing and su...