South Korea's impeached President Yoon Suk Yeol appeared at the Constitutional Court for the first time Tuesday, and vowed to cooperate with the judges who will decide whether to remove him from office.

The country was plunged into political chaos by Yoon's 3 December martial law declaration, which lasted just six hours before lawmakers voted it down.

They later impeached him, stripping him of his duties. He also became the first sitting South Korean president to be arrested in a criminal probe on insurrection grounds.

Thousands of protesters — both for and against Yoon — flocked to the Constitutional Court, which is holding hearings to decide whether to uphold his impeachment.

"I will respond to any questions or provide further remarks if necessary," Yoon told the judge.

Yoon was asked whether he had instructed top military commanders to "drag out" lawmakers from parliament to prevent them voting down his martial law decree.

"No," he told the judge.

Court spokesperson Cheon Jae-hyun told reporters that Yoon's legal team have requested to call "at least 24 individuals" as witnesses, including election-related officials.

Yoon, who remains head of state, is being held in a detention centre pending a criminal probe on charges of insurrection.

Yoon and his legal team have sought to justify his attempt to suspend civilian rule as a necessary measure due to election fraud, after the opposition won parliamentary elections by a landslide last year.

According to pool reporters, he appeared in court wearing a suit — not his standard-issue prison uniform, which he has been required to wear since he was formally arrested Sunday.

Yoon's legal team said he wanted to "personally appear to explain the circumstances surrounding the declaration of martial law."

Shortly after the hearing, the Corruption Investigation Office probing the martial law declaration said it had sent six investigators and a prosecutor to the detention centre to question Yoon, who has been refusing to submit to their questioning.

However the van escorting Yoon was seen travelling to a hospital in Seoul, instead of the detention centre.

The justice ministry said Yoon went for "a medical treatment," based on results of a medical exam the previous day.

Entirely incomprehensible

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

The lawyers prosecuting the case, who were selected by the parliament, slammed the comments made by Yoon and his lawyers as "largely contradictory, irrational, and unclear, making them entirely incomprehensible."

"If they continue to evade responsibility as they did today, it will only work against them in the impeachment trial and cause even greater disappointment among the public," they added.

Yoon stayed away from the first two hearings last week, but the trial — which could last months — will continue even if he is absent.

Riling public opinion

Yoon made his first court appearance on Saturday at a hearing on whether to extend his detention. When it was extended, hundreds of pro-Yoon protesters attacked the court building and scuffled with police officers. Dozens were arrested.

The impeached president's decision to start showing up at the Constitutional Court hearing is more about inflaming his die-hard supporters than helping the judicial process along, legal expert Kim Nam-ju said.

"Whether it's the legal representative speaking or Yoon himself speaking it's nearly the same, it's more about riling public opinion," Kim told AFP. 

But whatever Yoon's motives, "from the perspective of the Constitutional Court judges, hearing directly from the defendant is far more significant," he said. 

Yoon's presence will allow "the judges to confirm the facts firsthand."

Kim added that Yoon's refusal to engage with the criminal investigation into his martial law will not work in his favour overall, even if he continues showing up at the Constitutional Court.

"Refusing to comply with the warrant execution and declining to testify will gradually be considered as factors unfavourable to his case in the impeachment trial," said Kim.

"It shows they are not adhering to the legal framework."

Yoon has claimed the criminal probe is illegal and resisted arrest for weeks, vowing to "fight to the end."