Rally organizers said some 250,000 gathered in the capital's Gwanghwamun Square for a candlelight vigil to demand a swift decision on Park, according to the Yonhap news agency
. The rally marked the ninth consecutive week of such protests.
Lawmakers in South Korea's National Assembly voted overwhelmingly to impeach Park
on December 9, stripping away her executive powers. The Constitutional Court will now decide on whether she should be kicked out of office permanently, a process that could take up to 180 days.
Park, the country's first female leader, has been embroiled in a classified information scandal involving longtime friend Choi Soon-sil. She has denied wrongdoing but apologized for carelessness in her ties with Choi.
Prime Minister Hwang Kyo-ahn is acting President for the duration of the court's deliberation. Park has refused to give up her position, which enables her to remain immune to prosecution.
Investigators question Park confidante
Saturday's march came on the same day as the South Korean special prosecution investigating the scandal questioned Choi.
Choi and other former aides were charged in November with abuse of power and fraud.
Choi, who has been in custody since late October, is accused of using her personal relationship with the President for personal benefit. The 60-year-old arrived Saturday afternoon at the office of the special prosecutor in southern Seoul, according to Yonhap.
Lee Kyu-chul, a spokesman for the team of investigators, told a press briefing that Choi was questioned on charges of bribery and the transfer of embezzled assets abroad.
The special prosecutor has up to 100 days to investigate allegations that Park colluded with Choi and her aides to pressure big conglomerates to contribute 77 billion won ($64 million) to foundations set up to back her policy initiatives.
Choi accused of intervening in state affairs
CNN South Korean affiliate JTBC broke the news of the scandal in October after revealing it had found an abandoned computer of Choi's containing evidence she received secret documents and intervened in state affairs.
Local media and opposition parties accused Choi of abusing her relationship with Park to force big conglomerates to donate millions of dollars to two foundations they allege she had set up.
Choi is also accused of pressuring a local university into giving her daughter special treatment, including changing school regulations so she could get good grades without attending classes.