South Korea’s president helped a longtime friend extort money from corporations and allowed her to receive classified documents, prosecutors said Sunday, WSJ reported.

The claims are the first formal accusations of wrongdoing by Park Geun-hye, who becomes the only South Korean president to face a criminal investigation while in office. The scandal has paralyzed her government and triggered the nation’s largest-ever street demonstrations calling for her ouster.

“Based on our investigations, we judge President Park, to a significant extent, had an accomplice role in the crime,” Lee Young-ryeol, head of the Seoul Central District Prosecutors’ Office, said at a press conference Sunday.

A spokesman for the president’s office said the accusations were “built on imagination and conjecture,” and accused the prosecutors of political bias. He said Ms. Park and her staff would cooperate with a planned new investigation “to prove that the president is not guilty.”

Prosecutors on Sunday indicted Ms. Park’s friend, Choi Soon-sil, and two former presidential aides, following weeks of allegations in South Korean media that Ms. Choi used her ties to the president for financial gain and to interfere in government.

Ms. Park has twice apologized on national television for her ties to Ms. Choi but hasn’t directly addressed the allegations. Ms. Park has said she is willing to be questioned by prosecutors, a move that was expected last week but was delayed by her lawyer, citing the need to fully prepare.

Ms. Choi, who has been a friend of Ms. Park for about 40 years, has been detained by prosecutors since Oct. 31. Ms. Choi has said she helped the president with speeches, but denied other allegations against her.