Over the past days, tens of thousands of South Koreans have protested in the capital, Seoul, to demand the resignation of President Park over the corruption row.
Ms Choi, a long-time friend of Ms Park's, is the daughter of Choi Tae-min, a shadowy quasi-religious leader who was closely linked to Ms Park's father, then-president Park Chung-hee.
She is alleged to have pushed businesses to donate millions of dollars to foundations she controlled, helped choose presidential aides, and even picked the president's clothes.
Ms Park has since apologised on TV for allowing her long-standing friend inappropriate access to government policy-making. The president admitted she had let Ms Choi edit her speeches.
President Park has already replaced her prime minster, reshuffled her cabinet and dismissed several aides, but there are growing calls for her resignation or impeachment.

The scandal has left Ms Park with an approval rating of just 5%.
Choo Mi-ae, leader of the main opposition Democratic Party, said she did not believe the apology was genuine and called on her to accept a new prime minister recommended by parliament.
Ms Park became her country's first female president when she was elected in a close-run contest in December 2012.