Impeached South Korean President Yoon Suk Yeol was in court for the first time on Saturday for a crucial hearing that will decide whether to extend his detention as investigators probe his failed martial law bid.

Yoon's supporters rallied outside the court and scuffled with police as he arrived in a blue van, weeks after throwing the nation into chaos by attempting to suspend civilian rule.

The president's December 3 martial law bid lasted just six hours, with lawmakers voting it down despite him ordering soldiers to storm parliament to stop them.

Yoon was subsequently impeached by parliament and resisted arrest for weeks, holed up in his guarded residence until he was finally detained on Wednesday in a dawn raid.

South Korea's first sitting president to be detained, Yoon refused to cooperate during the initial 48 hours detectives were allowed to hold him.

But the disgraced president remains in custody after investigators requested a new warrant on Friday to extend his detention.

He decided to appear in court "with the intention of restoring his honour," the president's lawyer Yoon Kab-keun said before the hearing.

A spokesperson for Seoul Western District Court later confirmed that the hearing commenced with the president in attendance.

After reviewing the request to extend Yoon's detention, a judge is expected to announce her decision on Saturday night or early on Sunday.

Outside the court, crowds of Yoon's backers waving flags and holding "release the president" placards.

Police officers formed a chain to prevent them reaching the court entrance, which has been closed to the public since Friday evening over safety concerns.

If approved, as expected, the new warrant would likely extend Yoon's detention by 20 days.

This would give prosecutors time to formalise an indictment for insurrection, a charge that could see him jailed for life or executed if found guilty.

If indicted, the president would probably be detained for the duration of the trial. (AFP)