Choi, who has never had any official position in the country's administration, is suspected of having a long record of intervening in Park's decisions, including those directly involving South Korea's foreign and domestic policies. According to South Korean opposition newspaper The Hankyoreh, the president was fully controlled by Choi through Park's three senior advisers.
"I decided to join the candlelight rally today because this total nonsense involving President Park and Choi has reached well beyond its limit," said Shin Sook-hee, 40, who came from Suwon with her 10-year-old daughter.
Many of the people attending voted for president Park during in the 2012 election that brought her to power. According to media reports, there are also many students both from high schools and universities. Choi's daughter Chung Yoo-ra is accused of receiving special treatment during her admission process to attend Ewha Womans University, thanks to her mother's connection to President Park.
"We are out in the streets today because we feel the basic tenets of democracy have been shattered by the president herself," said Won Seung-jae, a high school senior from Incheon.
Choi voluntarily arrived in South Korea the same day to face interrogation, saying she is going to fully cooperate with the investigation, according to her legal representative Lee Kyung-jae.
President Park has ordered at least 10 presidential advisors to resign following the scandal. Three secretaries, who local media outlets call Park's closest aides, also stepped down. One of them reportedly delivered presidential speeches and documents to Choi regularly.
Park has offered apologies for the situation, saying that Choi only "gave her advice" during the initial period of Park's leadership.
According to some reports, it is expected that the whole government including the prime minister will be dismissed. However, it remains uncertain whether such measures will satisfy the public or whether the president herself will eventually be forced to resign.