Ousted South Korean president Yoon Suk Yeol will become the first leader to stand trial in a criminal case as hearings open on Thursday over his bid to impose martial law.

The 64-year-old former prosecutor has been in prison since he was arrested in January on charges of insurrection, for which he could be sentenced to life in prison or face the death penalty if found guilty. Criminal proceedings begin on Thursday at Seoul’s central district court.

Prosecutors have accused the suspended president of being the “ringleader of an insurrection”. His lawyers say the investigation lacked legitimacy from the start and have challenged the legality of his indictment, claiming it was within his power as head of state to declare martial law.

Separately, South Korea’s constitutional court is deliberating whether to formally remove Yoon from office following his impeachment by parliament in December. His 10th hearing in that case is scheduled for 3pm, just hours after he takes the stand in his criminal trial.

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, who is 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. 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 lasted only 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”.