Choi Soon-sil, a long-time friend of Ms Park, is accused of influence peddling and interfering in state affairs.
Ms Choi is alleged to have been involved in government business, despite having no security clearance.
It comes amid protests in Seoul demanding the president’s resignation.
Police said about 8,000 protesters took to the streets on Saturday after Ms Park said she had ordered 10 of her senior advisers to quit and admitted she had allowed Ms Choi to edit political speeches.
On Saturday, prosecutors raided the homes of several presidential aides, seizing computers and files belonging to the officials who are suspected of being Ms Choi’s accomplices.
Ms Choi is the daughter of shadowy religious cult leader Choi Tae-min, who was Ms Park’s mentor until his death in 1994.
Ms Park’s televised apology over the scandal last week failed to defuse the situation, only sparking widespread accusations of mismanagement.
The scandal has resulted in calls for her resignation from opposition parties and badly eroded her popularity before next year’s elections.
Ms Park, 64, became the first woman to lead South Korea after winning presidential elections in 2012.
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