Seoul (AFP) - Ousted South Korean leader Yoon Suk Yeol became the country’s first sitting head of state to stand trial in a criminal case as hearings opened Thursday on his bid to impose martial law.

The 64-year-old former prosecutor has been behind bars since he was arrested last month on charges of insurrection, for which he could be sentenced to life in prison or face the death penalty.

Criminal proceedings kicked off at 10 am (0100 GMT) Thursday at Seoul’s Central District Court, an AFP reporter in the room said.

With Yoon in attendance, the courtroom was packed and there was heavy security around the building.

A supporter of the ousted president dressed in a Captain America outfit was spotted outside the security perimeter.

Prosecutors have accused the suspended president of being the “ringleader of an insurrection”.

But his lawyers insist the investigation lacked legitimacy from the start and have challenged the legality of his indictment, given it was within his power as head of state to declare martial law.

Yoon’s lawyer Kim Hong-il described the trial as a “significant event in constitutional history”.

“The judiciary must serve as the stabilising force,” he told the court, warning that he was “witnessing a reality where illegality compounds illegality”.

He urged the court to “ensure that the defendant’s rights are not unjustly violated”.

Separately, South Korea’s Constitutional Court is deliberating whether to formally remove Yoon from office following his impeachment by parliament in December.

His tenth hearing in that case is scheduled for 3 pm.

- Top brass to testify -

Called to testify at the Constitutional Court are Han Duck-soo, who was also impeached as acting president following Yoon’s removal from office in December, and former senior intelligence official Hong Jang-won.

The head of South Korea’s National Police Agency Cho Ji-ho – also on trial on insurrection charges related to the martial law decree – has also been called as a witness.

But it is still not clear whether that impeachment hearing will be his last before the Constitutional Court’s eight judges go behind closed doors to deliberate his fate.

That process could take up to a fortnight or even longer.

Previously impeached presidents Park Geun-hye and Roh Moo-hyun had to wait 11 and 14 days, respectively, to learn their fates.

If Yoon is removed from office, the country must hold fresh presidential elections within 60 days.

Much of Yoon’s impeachment trial has centred on the question of whether he violated the constitution by declaring martial law, which is reserved for national emergencies or times of war.

His decree only lasted around six hours as the opposition-led parliament defied troops to vote it down. But it has plunged the democracy into months of political turmoil with protests, two impeachments and a surge of online disinformation.

Yoon’s lawyers told reporters last week that his martial law declaration was “an act of governance and cannot be subject to judicial review”.