South Korea president's friend Choi Soon-sil detained

The woman at the centre of the political scandal embroiling President Park Geun-hye has been placed 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 questioned for hours on 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 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, and it's possible an unexpected event could occur if she is released."

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Choi flew back to Seoul on Sunday from Germany to submit to herself for questioning and was surrounded 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 shoe as she struggled through the scrum to the Seoul district prosecutor's office on 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 on Tuesday wearing a 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.

Public anger

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 caused a media frenzy in South Korea, with lurid reports of religious cults and shamanistic rituals.

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 down her approval ratings.

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.

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

Park on Sunday carried out a partial reshuffle of key aides accused of being linked to Choi.

She is also considering calls from her ruling Saenuri Party to form a neutral multi-party cabinet to restore public trust and national unity.

The main opposition Democratic Party of Korea has stopped short of demanding Park's resignation.

But it is refusing to begin cross-party talks on how to end the crisis until the investigation into Choi has run its course.

Under South Korea's constitutional law, an incumbent president is exempt from being submitted to prosecution for any criminal offence.