South Korean investigators arrested Choi Soon Sil, the woman at the centre of the political scandal involving President Park Geun Hye, Yonhap News Agency reported.
Choi Soon Sil, a close confidante the president, has been accused of fraud and meddling in state affairs, triggering widespread anger among South Koreans.
The prosecutors are probing if she gained access to classified documents using her friendship with the president and made personal monetary gains through non-profit foundations.
South Korean media says Choi collected as much as 80 billion won ($92 million) in purported donations from businesses for her non-profit organisations.
Choi Soon Sil has been placed under emergency detention without a warrant, to prevent her from destroying evidence, Yonhap said on Tuesday.
The woman is currently at a Seoul detention facility, the agency said. Choi had earlier revealed she had access to the newly elected president's speeches but denied she had received any classified documents. President Park also admitted Choi had edited some of her speeches.
In a sensational revelation to media Choi later said she committed sins that "deserved death." "Please, forgive me. I'm sorry," she said.
According to local media, Choi pressured businesses to donate money to the Mir and K-Sports foundations she controls.
On Sunday, with the scandal thickening, Park carried out a partial reshuffle of her key aides and ordered her secretariat to hand in their resignations. The presidential spokesman said she has already accepted resignations from her chief of staff and four senior presidential secretaries.
However, the main opposition Democratic Party of Korea has raised strident criticism of the president , saying her administration is heavily reliant on a close circle of personal friends and longtime confidantes.