SEOUL, South Korea — In one of the largest anti-government protests in recent decades, hundreds of thousands of South Koreans filled central Seoul on Saturday to demand the resignation of President Park Geun-hye, whose administration has been paralyzed by a scandal involving an unofficial presidential adviser.

“You are surrounded! Park Geun-hye, come out and surrender!” protesters chanted, their voices reverberating through the center of the capital.

The main boulevard that faces the presidential offices and residence, known as the Blue House, shimmered with light from candles held by the protesters. Police buses formed a barricade to block protesters from getting too close to the presidential compound.

Outrage at Ms. Park has grown in recent weeks over allegations that she let a private adviser manipulate her and extort large sums from Korean companies.