"The special counsel's team is no longer democratic… They have been forcing me to confess that I colluded with President Park," Choi said on the way to the investigation team's office, as quoted by the Yonhap news agency.
Lee Kyu-chul, the investigation team's spokesman, has denied these claims, adding that the probe would continue in accordance with the law.
Choi has refused to be questioned by the investigators for nearly a month.
Political scandal around the South Korean president broke out in late October 2016, when media reported that Park had allowed her friend Choi, who held no official post, to edit her speeches thus letting the "shadow adviser" influence the country's policy. Moreover, Choi is accused of pressuring big South Korean corporations and extorting money from them for her non-commercial funds.
On December 9, the South Korean parliament voted to impeach the president in the wake of the scandal. The court has six months to decide upon the validity of the impeachment. If it is proved valid, presidential elections will be held within two months.