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

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

After the court extended his detention around 3 a.m. (1800 GMT Saturday), Yoon’s supporters smashed windows and doors as they rushed inside.

AFP journalists saw hundreds of police officers charge into the court, arresting dozens and condemning the incident as an “intolerable illegal and violent” act.

The incident is the latest development in South Korea’s spiraling political crisis since December 3, when Yoon declared martial law and sent troops to parliament. His attempt to suspend civilian rule lasted just six hours after lawmakers defied soldiers to vote it down. They later impeached Yoon, suspending him from office.

Yoon has vowed to "fight to the end" despite a Constitutional Court ruling on his impeachment and a criminal probe on insurrection charges that led to his detention.

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

The president thanked his supporters — including evangelical Christians and right-wing YouTubers — for their "passionate patriotism" in a message through his lawyers on Friday.

His backers have claimed Yoon was justified in imposing martial law due to alleged election fraud in legislative polls won by the opposition last year, though they provide no evidence. They frequently wave American flags and adopt the "stop the steal" rhetoric associated with U.S. president-elect Donald Trump, whose supporters stormed the Capitol to try to overturn his election defeat.

After the Seoul court incident, acting police chief Lee Ho-young said the force would “thoroughly investigate right-wing YouTubers if they were involved in this violent break-in.”

'Create burdens'

Yoon's lawyer, Seok Dong-hyeon, criticized the court decision and warned the president’s supporters not to escalate the situation.

"This is likely not what President Yoon desires," Seok said in a statement, adding that violence could "create burdens" for the president’s future trials.

Yoo Jung-hoon, a lawyer and political columnist, said attacking a court was "unprecedented" in South Korea and that 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."

With Yoon back behind bars after his court appearance Saturday, prosecutors are expected to formalize a criminal indictment for insurrection. The disgraced leader, who has refused to answer investigators' questions, could face life in prison or execution if convicted.

Meanwhile, Yoon has been absent from the parallel probe at the Constitutional Court, which is considering whether to uphold his impeachment. If the court rules against him, Yoon will formally lose the presidency, and elections will be called within 60 days.