Seoul (AFP) - Supporters of South Korean President Yoon Suk Yeol stormed a Seoul court Sunday after a judge extended the impeached leader’s detention over his ill-fated attempt to impose martial law.

Yoon said he was “deeply shocked and saddened” by the pre-dawn attack on the court, where his supporters smashed windows and doors.

Tens of thousands of people had gathered outside the Seoul Western District Court in a show of support Saturday for the president, who became South Korea’s first sitting head of state to be arrested in a dawn raid this week.

AFP journalists saw hundreds of police officers charge into the court, with the force arresting dozens and denouncing an “intolerable illegal and violent incident”.

The incident is the latest episode in South Korea’s spiralling political crisis which erupted on December 3, when Yoon declared martial law and dispatched troops to parliament.

His attempt to suspend civilian rule lasted just six hours after lawmakers defied soldiers to vote it down. They later impeached the president, suspending him from duty.

Yoon vowed Sunday to “persist, no matter how long it takes, in rectifying any injustices”, despite facing a Constitutional Court ruling on his impeachment and the criminal probe that has seen him detained.

In announcing investigators could hold Yoon for a further 20 days, the Seoul court told AFP there were concerns he could destroy evidence if released.

The president said he would attempt to show the “purpose and legitimacy of the martial law declaration”, in a statement issued Sunday through his lawyers.

His backers have claimed Yoon’s decision was justified due to election fraud in legislative polls won last year by the opposition, for which they present no evidence.

They frequently wave American flags and have adopted the “stop the steal” rhetoric associated with US President-elect Donald Trump, whose supporters stormed Washington’s Capitol to try to overturn his earlier election defeat.

Acting police chief Lee Ho-young said the force would “thoroughly investigate right-wing YouTubers if they were involved in this violent break-in” after the attack was livestreamed.

- ‘Anger of many citizens’ -

After the attack on the court, Yoon said he saw the “deep frustration and anger of many citizens”.

His lawyer Seok Dong-hyeon slammed the judge’s decision, while also warning the president’s supporters not to escalate the situation.

Such violence could also “create burdens” for the president’s future trials, the lawyer said in a statement.

Yoo Jung-hoon, a lawyer and political columnist, said attacking a court was “unprecedented” in South Korea and those involved would likely face jail time.

The judge was expected to keep the president in prison “given the myriad evidence supporting the insurrection charges”, Yoo told AFP.

“The court also placed significant weight on Yoon’s attempts to destroy evidence” as a head of state, he added.

With Yoon back behind bars after his court appearance on Saturday, prosecutors are due to formalise a criminal indictment for insurrection.

The disgraced leader, who has said he does not accept the legality of the probe, could be jailed for life or executed if found guilty.

He refused to attend questioning Sunday, said the Corruption Investigation Office which plans to summon him again at 10:00 am (0100 GMT) Monday.

Yoon has meanwhile been absent from the parallel probe at the Constitutional Court, which is considering whether to uphold his impeachment.

If that court rules against him, Yoon will formally lose the presidency and elections will be called within 60 days.