Prosecutors are investigating allegations that Choi Soon-sil used her friendship with President Park Geun-hye to influence state affairs by gaining access to classified documents and benefited personally through non-profit foundations, another prosecution official said earlier.
Park is in the fourth year of a five-year term and the crisis threatens to complicate policymaking during the lame-duck period that typically sets in toward the end of South Korea's single-term presidency.
Worried that Choi may be a flight risk and could destroy evidence, prosecutors placed her under emergency detention without a warrant, South Korea's Yonhap News Agency reported.
The prosecution official confirmed that she had been arrested and taken to a Seoul detention facility, declining provide further details.
Choi Soon-sil (C) arrives at the Seoul Central District Prosecution Office for questioning in Seoul, South Korea, 31 October 2016. (AAP) Source: AAP
On Tuesday morning, Choi arrived by prison bus at the prosecutors' office, escorted by correctional officers, according to a live broadcast by Yonhap News TV.
Choi's lawyer was not immediately available for comment early on Tuesday.
Under South Korean law, a suspect can be held under emergency arrest without a warrant for up to 48 hours. A longer detention requires an arrest warrant issued by a court.
Choi begged forgiveness when she arrived to meet prosecutors earlier on Monday.
In an interview with South Korea's Segye Ilbo newspaper published on Thursday, Choi said she received drafts of Park's speeches after Park's election victory but denied she had access to other official material, or that she influenced state affairs or benefited financially.
Park said last week she had given Choi access to speech drafts early in her term and apologised for causing concern among the public.
Thousands of South Koreans rallied in Seoul on Saturday night demanding Park's resignation over the scandal.
