SEOUL, South Korea — The shadowy woman at the center of President Park Geun-hye’s worst political scandal apologized on Sunday for her “wrongdoings.” Hours later, Ms. Park fired her chief of staff and seven other presidential aides in an effort to regain public trust, a day after thousands of South Koreans took to the streets to call for her removal from office.

Choi Soon-sil, a longtime associate widely seen here as a shamanlike adviser to Ms. Park, returned to South Korea on Sunday from Europe, where she had been in hiding since the scandal erupted weeks ago. Ms. Choi’s lawyer, Lee Gyeong-jae, said she would present herself to prosecutors for questioning on her murky ties with Ms. Park, which are at the heart of the president’s troubles.

“She apologizes deeply for causing the people humiliation and despair,” Mr. Lee said of Ms. Choi at a news conference.

Mr. Lee said Ms. Choi also apologized for “her wrongdoings,” but he did not elaborate.

Ms. Park has been accused of letting Ms. Choi, a private citizen with no security clearance or background in policy making, advise her on crucial state affairs. Ms. Choi, 60, has also been accused of using her influence with Ms. Park to plant her associates in the government, including the presidential office, and to coerce big businesses to donate millions of dollars each to the two foundations she controls.