"There are many parts that are unfair to me," Choi said after denying the allegations, as quoted by the Yonhap news agency.
Former presidential aides An Chong-bum and Jeong Ho-seong, who are accused of collaborating with Choi, were also at the hearing. The former denied charges of helping Choi's foundations extort money from corporations, while the latter's lawyer delayed the decision on whether or not to plead guilty.
The Constitutional Court of South Korea also resumed its hearings on impeaching Park after holding a nine-minute hearing on Tuesday, with the president failing to make an appearance. Park's lawyers said she was unlikely to attend the trial.
The Constitutional Court has six months to decide upon the validity of the parliament's impeachment of Park. If the impeachment is recognized as valid, presidential elections should be called within two months.