New Delhi, Sept. 15 -- Serbian President Aleksandar Vucic is escalating his crackdown on protests that have shaken his populist rule in recent months. What began as a small, student-led campaign against corruption has snowballed into one of the most turbulent protest waves in the Balkan country in a quarter of a century.

Rights groups and Vucic's political opponents have warned of increasingly brutal tactics aimed at silencing a movement that has become the biggest challenge yet to his decade-long grip on power.

Last week, tanks rolled through the capital, Belgrade, in preparation for a military parade on Sept. 20. If the parade becomes a flashpoint for unrest, protesters fear the military could remain on the streets.

Vucic's nationali...