It remained unclear exactly for what she was apologising as she has denied charges of corruption.
“I have committed an unpardonable crime,” said Choi as she arrived at the prosecutors’ offices.
However, it remained unclear exactly for what she was apologising but Choi has, in recent days, denied charges of corruption.
Officials are trying to determine whether Choi should be accused of corruption and exercising undue influence.
Park allegedly let Choi, who holds no official job or security clearance, view government documents and weigh in on personnel decisions.
Choi also stands accused of using her ties to the president to raise funds with which she founded two private institutes.
She returned to South Korea on Sunday, after a long stay in Germany, to speak to the prosecutors.
Park has admitted to relying upon Choi’s counsel during her presidential campaign and since taking office in 2013, primarily in the form of access to speeches before they were delivered.
The scandal has seen Park’s political support evaporate, with demonstrations Saturday calling for her to step down drawing thousands.
The one weekend poll saw her support at 17 per cent.
The two have been friends for years.
Choi is the daughter of one of Park’s earliest political backers.
She has denied any wrongdoing, saying that she only helped the president write speeches and that she would accept any punishment if she is found guilty of criminal acts.