SEOUL— Hundreds of thousands of South Koreans filled central Seoul on Saturday to demand President Park Geun-hye’s resignation, in what appeared to be the largest turnout yet in a series of weekly protests against the embattled leader.

Despite cold weather and the first snow of the season, a crowd that organizers estimated at 1.5 million gathered to denounce Park, who has been accused by prosecutors of helping a friend commit extortion and is facing the possibility of impeachment within weeks.

The police estimate of the turnout was much lower, at 260,000, but the crowd seemed larger than at an enormous rally in the capital two weeks earlier. It was the fifth consecutive Saturday marked by a large protest against the president.

Prosecutors have identified Park as a criminal accomplice in the case of Choi Soon-sil, an old friend who has been charged with using her influence to coerce businesses into donating large sums to foundations that Choi controls. In news reports, Park has also been accused of letting Choi wield undue influence in state affairs.