Ms Choi, a close friend of President Park Geun-hye, is charged with abuse of power and attempted fraud.

Separately, the Constitutional Court began considering President Park's impeachment over the scandal.

That trial, where the court must ratify or overturn a parliamentary impeachment vote, began despite Ms Park's absence.

Proceedings opened on Tuesday but were curtailed because Ms Park did not attend. On Thursday the court went ahead anyway.

Her lawyer has previously said Ms Park will not be in court "unless there are special circumstances", Yonhap news agency reported.

Both cases centre on allegations President Park gave Ms Choi unauthorised access to government decisions and allowed her to exploit their close relationship to solicit money from corporations for foundations from which she benefitted.

Both women have apologised but deny committing criminal offences.

In court, Ms Choi formally denied the charges against her. "I am facing much unfairness," she reportedly said.

Her lawyer told the court there had been "no collusion" between Ms Choi and the president.

"The accused has not sought one bit of financial gain for herself since the day the foundations were established," Lee Kyung-jae said.
Extradition

The scandal has rocked South Korea, sparking several huge demonstrations.

While the impeachment process continues, Ms Park remains formally president but with most of her powers handed to the prime minister, a role that normally has little executive authority.

Separately, Yonhap news agency reports that South Korea has sent a formal extradition request to Denmark for the daughter of Ms Choi, several days after saying they were working on it.

Chung Yoo-ra, a former national equestrian rider, was arrested in Denmark on Sunday and later appeared in court accused of overstaying her visa.

Part of the investigation into Ms Choi's activities relates to a gift horse from South Korean conglomerate Samsung to Ms Choi, allegedly for Ms Chung's training.

The prestigious Ewha Women's University in Seoul is also accused of giving Ms Chung a place - she has since left - because of her mother's connections.