South Korea's prosecutors indicted President Yoon Su-yeol on Sunday on charges of leading an insurrection with his short-lived imposition of martial law on December 3, the main opposition party said.

"The prosecution has decided to indict Yoon Suk-yeol, who is facing charges of being a ringleader of insurrection," Democratic Party spokesman Han Min-soo told a press conference. "The punishment of the ringleader of insurrection now begins finally."

The indictment was also reported by South Korean media.

Anti-corruption investigators last week recommended charging the jailed Yoon, who was impeached by parliament and suspended from his duties over the incident.

Yoon's lawyers had urged the prosecutors to immediately release the him from what they call illegal custody.

Under criminal investigation, he has been in custody since becoming the first sitting president to be arrested on January 15.

Insurrection is one of the few criminal charges from which a South Korean president does not have immunity. It is punishable by life imprisonment or death, although South Korea has not executed anyone in decades.

Yoon and his lawyers argued at a Constitutional Court hearing last week in his impeachment trial that he never intended to fully impose martial law but had only meant the measures as a warning to break political deadlock.

In parallel with his criminal process, the top court will determine whether to remove Yoon from office or reinstate his presidential powers, with 180 days to decide.

South Korea's opposition-led parliament impeached Yoon on December 14, making him the second conservative president to be impeached in the country.

Yoon rescinded his martial law after about six hours after lawmakers from the main opposition party, confronting soldiers in parliament, voted down the decree.

Soldiers equipped with rifles, body armour and night-vision equipment, were seen entering the parliament building through smashed windows during the dramatic confrontation. (Reuters)