In the wake of mass street protests in Seoul and other cities to demand Park's resignation, Choi Soon-Sil - who has denied any criminal wrongdoing - submitted to prosecutors in Seoul a day after flying back from Germany.
Park and Choi have been close friends for 40 years. The precise nature of that friendship lies at the heart of the current scandal which has triggered a media frenzy in South Korea, with lurid reports of religious cults and shamanistic rituals.
The media has portrayed the 60-year-old Choi as a Rasputin-like figure, who wielded an unhealthy influence over Park and interfered in government policy despite holding no official post and having no security clearance.
Suggestions that Choi vetted presidential speeches and was given access to classified documents has exposed Park to public anger and ridicule and, with just over a year left in office, pushed her approval ratings off a cliff.
South Korean President Park Geun-hye speaks as she offers a public apology in Seoul, South Korea, Tuesday, Oct. 25, 2016. (AAP) Source: AAP
'Deadly sin'
Televised footage of Choi's arrival at the Seoul Central District Prosecutors' Office showed a distraught-looking Choi - dressed head to toe in black with her face covered with a hat and scarf - as she stepped out of a black sedan to face hundreds of reporters.
Choi did not say a word as she sobbed and shoved her way into the building past reporters and protesters with placards that read "Arrest Choi Soon-Sil! Impeach Park Geun-Hye!"
"Please forgive me. I have committed a deadly sin," Choi was quoted as saying by Yonhap news agency after stepping inside the building.
A task force, led by the head of the Seoul prosecutors' office, has been set up to investigate the leak of presidential documents and whether Choi meddled in state affairs.
Choi Soon-sil is surrounded by media upon her arrival at the Seoul Central District Prosecutors' Office in Seoul, South Korea, Monday, Oct. 31, 2016. Source: AAP
Choi has also been accused of using her relationship with the president to coerce corporate donations to two non-profit foundations, and then siphoning off funds for her personal use.
"We hope that the various allegations can be thoroughly verified," presidential spokesman Jung Youn-Kuk told reporters.
Choi is the daughter of a late shadowy religious leader and one-time Park mentor called Choi Tae-Min, who was married six times, had multiple pseudonyms and set up his own cult-like group known as the Church of Eternal Life.
Choi Tae-Min befriended a traumatised Park after the 1974 assassination of her mother, whom he said had appeared to him in a dream, asking him to help her daughter.
He became a long-time mentor to Park, who subsequently formed a close bond with Choi Soon-Sil that endured after Choi Tae-Min's death in 1994.
Choi Soon-Sil's ex-husband served as a top aide to Park until her presidential election victory in 2012.
A public apology by Park, in which she acknowledged seeking limited advice from Choi, did little to assuage public outrage and she has struggled to draw a political line under the crisis.
Political paralysis
Park carried out a partial reshuffle of her key aides on Sunday and is considering calls from her ruling Saenuri Party to form a neutral multi-party cabinet to restore public trust and national unity.
In a message sent to reporters, one of her senior advisers who stepped down in the reshuffle described Park as "lonely and sad".
The main opposition Democratic Party of Korea has stopped short of demanding the president's resignation, but is refusing to begin cross-party talks until the investigation into Choi has run its course.
Analysts say the scandal could paralyse Park's administration, underlining her lame-duck status before presidential elections in December next year.
And all this at a time of slowing economic growth, rising unemployment and elevated military tensions with North Korea.
A protester wears a photograph of Choi Soon-Sil next to a protester holding a banner reading 'President Park Geun-Hye Resignation' in Seoul, 26 October 2016. (AAP) Source: AAP
In his Monday briefing, presidential spokesman Jung stressed that the ongoing political uncertainty would not be allowed to open even the "slightest crack" in the country's defence readiness.
