Impeached South Korean President Yoon Suk Yeol was released from detention on Saturday after a court voided his arrest on procedural grounds – but he remains under investigation over his declaration of martial law.

The suspended president, who was detained in a dawn raid in January on insurrection charges over his December 3 attempt to subvert civilian rule, walked out of the detention centre smiling, before bowing deeply before a small crowd of cheering supporters.

"I bow my head in gratitude to the people of this nation," Yoon said in a statement released through his lawyers.

A day earlier, a court had cancelled his arrest warrant on technical and legal grounds – a decision that prosecutors probing Yoon said in a statement on Saturday was "unjust."

Yoon was freed after the prosecutors waived their right to appeal the court verdict, which was specifically about technical details of his detention on criminal charges.

Yoon also faces a separate Constitutional Court ruling on whether to uphold his impeachment and formally strip him from office, with the judges' decision expected any day.

Prosecutors said that "given the Constitutional Court's ruling and related considerations, the Prosecutor General has instructed the team to actively present its arguments before the trial court instead" of appealing Yoon's release from detention.

South Korea must hold a fresh presidential election within 60 days if Yoon is removed.

The criminal case against him will continue even if he is formally stripped of office. (AFP)