South Korea's impeached president, Yoon Suk Yeol, refused to be questioned by investigators on Monday under a probe into whether he committed insurrection, as dozens of his supporters faced being charged over a violent rampage on a court building.

Authorities said security was being beefed up at the Seoul Detention Center where Yoon is being held as a pretrial inmate and at the Constitutional Court, which is holding an impeachment trial to decide whether to permanently remove him from office.

Yoon planned to attend future impeachment trial hearings, including one on Tuesday, one of his lawyers, Yoon Kab-keun said.