South Korean prosecutors demanded Wednesday a 15-year prison term for former first lady Kim Keon Hee for alleged stock fraud and corruption crimes.

Kim, the wife of disgraced ex-president Yoon Suk Yeol, was arrested in August and is under investigation for an alleged stock manipulation scheme and for accepting gifts from a religious organisation widely seen as a cult.

She has also been accused of meddling in parliamentary elections.

Prosecutors said the 53-year-old had "stood above the law" and colluded with the Unification Church to "(undermine) the constitutionally mandated separation of religion and state". 

"This collapsed both the fairness of elections and the system of representative democracy that forms the foundation of national governance," they added.

They asked the court to hand her 15 years in jail and fine her two billion won ($1.37 million).

In her final testimony, Kim said the allegations were "deeply unjust".

"Yet when I consider my role and the responsibilities entrusted to me, it seems clear that I have made many mistakes," Kim said.

While there was "room to dispute" the allegations, "I sincerely apologise for the great discourtesy I have caused the public," she added.

Kim's final hearing came a year to the day that her husband Yoon declared martial law in a short-lived bid to suspend civilian rule that plunged South Korea into political turmoil.

Yoon was arrested earlier this year on insurrection charges, which he denies. It marked the first time that a South Korean former president and first lady had both been detained.

The court is set to sentence Kim on January 28.