South Korea's parliament has passed a law that would allow a special prosecutor to investigate a corruption scandal threatening the presidency of Park Geun-hye.

The opposition-controlled National Assembly also on Thursday voted for a parliamentary investigation into allegations that Park allowed a secretive confidante to manipulate power from the shadows and amass an illicit fortune.

The law on an independent investigation comes as state prosecutors prepare to indict Park's friend by Sunday. Prosecutors are also seeking to question Park but her lawyer has asked for more time.

Park has apologized for letting her friend Choi Soon-sil edit some of her draft speeches and for the public anger caused by the scandal. But she has not commented on the most damning accusations that she let Choi manipulate government affairs.