The woman at the centre of the snowballing political scandal engulfing President Park Geun-Hye has been put under emergency detention after prosecutors said she was "unstable" and a flight risk.

Choi Soon-Sil, who faces allegations of fraud and meddling in state affairs over her decades-long friendship with Park, was grilled for hours Monday after she returned to the country and handed herself in following mass street protests.

"There is a possibility of Choi trying to destroy evidence as she is denying all the allegations," a prosecution official told the Yonhap news agency, explaining the decision to hold her for 48 hours.

"She has fled overseas in the past, and she doesn't have a permanent address in this country, making her a flight risk.

"She is also in an extremely unstable psychological state," the official added.

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.

She flew back to Seoul Sunday from Germany and was mobbed by hundreds of journalists and angry protesters waving placards demanding her arrest.

Dressed from head to toe in black, Choi lost her hat, sunglasses and one Prada shoe as she struggled through the scrum to the Seoul District Prosecutor's Office Monday.

"Please forgive me. I have committed a deadly sin," Choi said after she made it inside the building, Yonhap reported.

After a night in detention, she was escorted back to the prosecutors' office early Tuesday wearing prison uniform for another round of questioning -- which could last for days, Yonhap said.

Prosecutors have to decide whether to seek a warrant to formally arrest Choi before the emergency detention period expires.

-- 'Help Choi die' --Park and Choi have been close friends for 40 years. The precise nature of that friendship lies at the heart of the scandal which has triggered a media frenzy in South Korea, with lurid reports of religious cults and shamanistic rituals.

Choi is the daughter of late shadowy religious leader 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.

He befriended a traumatised Park after the 1974 assassination of her mother -- whom he said had appeared to him in a dream -- becoming a mentor to Park, who subsequently formed a close bond with his daughter.

Suggestions that Choi vetted presidential speeches and was given access to classified documents have exposed Park to public anger and ridicule and, with just over a year left in office, pushed her approval ratings off a cliff.

According to a survey published Tuesday, Park's approval rating was 9.2 percent, with 67 percent of voters saying she should step down.

Choi has also been accused of using her relationship with the president to coerce corporate donations to two non-profit foundations, and then siphon off funds for personal use.

Park issued a public apology last week, acknowledging seeking limited advice from Choi on her speeches.

But it did little to assuage public outrage, with mass street protests erupting in Seoul and other cities to demand Park's resignation.

On Tuesday, a man rammed an excavator into a key building belonging to South Korea's prosecution service in Seoul.

The 45-year-old was detained and later told police interrogators that he had carried out the attack in a bid "to help Choi die as she said she committed a deadly sin", according to Yonhap.

The Hankyoreh newspaper reported Tuesday that Choi had been a frequent visitor to the presidential Blue House since Park took office in 2013 -- something Park's administration has staunchly denied.

Park carried out a partial reshuffle of her key aides Sunday and is considering calls from to form a neutral multi-party cabinet to restore public trust.

Analysts say the scandal could paralyse Park's administration, underlining her lame-duck status ahead of presidential elections in December next year.