South Korea's former first lady accepted lavish bribes including fine art, jewellery and designer handbags, and meddled in state affairs, prosecutors say.

Kim Keon Hee, the wife of disgraced ex-president Yoon Suk Yeol, was arrested in August and is under investigation for alleged stock manipulation and receiving gifts from the cult-like Unification Church.

She has also been accused of meddling in parliamentary elections.

In an announcement concluding their investigation, prosecutor Min Joong-ki said South Korea's institutions were "severely undermined by abuses of power" by Ms Kim.

The special prosecutor's investigations came amid a year-long probe into Mr Yoon's brief imposition of martial law last year and related scandals linked to the once-powerful couple.

Ms Kim allegedly "illegally intervened in state affairs behind the scenes, beyond the public's view," prosecutor Kim Hyung-geun said.

Bribery allegations

Prosecutors alleged Ms Kim accepted bribes from businesses and politicians totalled up to 377.25 million won ($390,000).

Investigators alleged Ms Kim had received two Chanel bags and a Graff necklace from the leader of the Unification Church.

She also allegedly received luxury jewellery, a painting by famed South Korean minimalist painter Lee Ufan, a Dior handbag and a wristwatch.

Mr Yoon denied that he was aware of these transactions when pressed by investigators, "a claim that many find difficult to accept", prosecutor Kim Hyung-geun said.

Prosecutors called for a 15-year prison term for the former first lady in early December, saying she had "stood above the law" and colluded with the Unification Church to undermine the "constitutionally mandated separation of religion and state".

They also asked the court to fine her 2 billion won.

The former first lady, who is in detention, has denied all the charges.

In her final testimony, she 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," she said.

Political turmoil

Mr Yoon declared martial law last December in a short-lived bid to suspend civilian rule that plunged South Korea into political turmoil.

He was removed from office and then arrested this year on insurrection charges, which he denies, the first time that a South Korean former president and his wife had both been detained.

A lower court ruling on Ms Kim is expected on January 28.

The prosecution team has also indicted Unification Church leader Han Hak-ja, now on trial, after the religious group was suspected of giving Ms Kim valuables, including two Chanel bags and a diamond necklace, as part of its efforts to win influence.