South Korea's impeached president Yoon Suk Yeol will be jailed for five years over abuse of power, obstructing justice and falsifying documents in relation to his failed martial law bid in 2024.

This is the first of the verdicts in four trials linked to his shock martial law decree. Although short-lived, the move triggered nationwide turmoil, sparking protests as MPs rushed to the national assembly to overturn Yoon's decision.

Yoon's actions "plunged the country into political crisis", a judge said on Friday, noting that Yoon had "consistently shown no remorse".

Friday's ruling offers clues as to how the rest of Yoon's trials could go. His string of charges range from abuse of power to campaign law violations.

The most serious charge is insurrection, for which prosecutors have demanded the death penalty. The verdict in that trial is expected in February.

About 100 supporters of Yoon had gathered outside the courthouse on Friday to watch the livestreamed proceedings projected on a large screen.

Some of them held red banners that read: "Yoon, again! Make Korea great again". Several could be heard yelling at the judge as he delivered the guilty verdicts, while others looked solemn.

On Friday, Yoon was convicted of using presidential bodyguards to prevent his arrest; failing to consult his entire cabinet before declaring martial law; as well as drafting and then destroying a falsified document claiming the martial law bid had been endorsed by the prime minister and defence minister.

"The accused has the duty to safeguard the constitution and law but turned his back on them," the judge said.

Prosecutors had sought a 10-year jail term for the charges in Friday's ruling. Both sides have seven days to appeal.

Yoon denied the charges, arguing that the arrest warrant itself was invalid and that the letter of the law does not require him to consult every member of the cabinet before exercising emergency power.

He had claimed across his trials that investigators had no legal basis to probe and arrest him in the first place. Most of the accusations against him are invalid because there was no procedural lapse when he declared martial law, he argues.

South Korea's courts often grant leniency when the accused accepts guilt or responsibility. But prosecutors argue that Yoon's lack of remorse is grounds for an even more severe penalty.

Park Geun-hye, who was sentenced to 20 years in prison for abuse of power and bribery in 2021, was the last former president to be jailed for a criminal charge. But she was pardoned and released from prison shortly after.

Six months after Yoon's martial law attempt, voters elected opposition leader Lee Jae Myung in a decisive victory .

But Yoon's trials bring the spotlight back to deep divisions in South Korea, where the former president retains strong supporters who see him as a martyr. According to a survey conducted last December, nearly 30% of South Koreans did not believe that Yoon's martial law declaration amounted to an insurrection.

While his martial law attempt brought tens of thousands of protesters onto the streets, it also saw his supporters show up in counter protests, though in smaller numbers.

Additional reporting by Koh Ewe