Former South Korean dictator Chun Doo-hwan went on trial Monday for defaming an activist priest who documented a brutal crackdown by his troops on pro-democracy demonstrators.

An uprising in the southern city of Gwangju against Chun's military government four decades ago ended in a bloodbath by martial law troops that left around 200 people dead or missing, according to official figures.

Activists say the toll may have been three times as much.

The 89-year-old Chun remains hugely divisive in South Korea and is accused of defaming late priest Cho Chul-hyun, who repeatedly testified that helicopters had opened fire on civilians in Gwangju.

In a 2017 memoir, Chun accused Cho of lying and called him "Satan in a mask".

"To my knowledge, there were no firings from the helicopters," it cited him as saying, and that doing so would have caused "mass casualties".

"I don't think that our country's sons, whether they be lieutenant or captain-level pilots, would have taken such actions."

Libel is a criminal offence in South Korea and if convicted, Chun could face up to two years in prison or five million won ($4,000) in fines.

Some far-right conservatives in South Korea continue to condemn the Gwangju uprising as a Communist-inspired rebellion, while liberal President Moon Jae-in has reopened investigations into the military's actions.

Chun seized power in a military coup in 1979 and ruled South Korea with an iron fist until 1988, stepping down after mass demonstrations forced him to agree to the restoration of democracy.

In 1996, he was convicted of treason and bribery and sentenced to death. But the country's top court commuted the penalty to life in prison and he was released the following year on a presidential pardon.