SEOUL, South Korea — Tens of thousands of South Koreans poured into the streets of downtown Seoul on Saturday, using words including “treason” and “criminal” to demand President Park Geun-hye step down amid an explosive political scandal.

The protest, the largest anti-government demonstration in the capital in nearly a year, came a day after Park apologized on live television amid rising suspicion she allowed a mysterious confidante to use power from the shadows.

Holding banners, candles and colorful signs that read “Park Geun-hye out” and “Treason by a secret government,” a sea of demonstrators filled a large square in front of an old palace gate and the nearby streets, singing and thunderously applauding speeches calling for the ouster of the increasingly unpopular president.

They then shifted into a slow march in streets around City Hall, shouting “Arrest Park Geun-hye,” ”Step down, criminal” and “We can’t take this any longer,” before moving back to the square and cheering on more speeches that continued into the night.

“Park should squarely face the prosecution’s investigation and step down herself. If she doesn’t, politicians should move to impeach her,” said Kim Seo-yeon, one of the many college students who participated in the protest.

“She absolutely lost all authority as president over the past few weeks,” he said.

Earlier in the week, prosecutors arrested Choi Soon-sil, the daughter of a late cult leader and a longtime friend of Park, and detained two former presidential aides over allegations that they pressured businesses into giving $70 million to two foundations Choi controlled.

Police estimated the crowd at 45,000, although protest organizers claimed 200,000 people.