SEOUL, South Korea — Impeached South Korean President Yoon Seok-yeol refused to answer questions Monday over an investigation into whether he committed an insurrection, as dozens of his supporters faced arrest over a violent attack on a court building.

Authorities said security was being stepped up at the Seoul Detention Center, where Yoon is being held, and at the Constitutional Court, where an impeachment trial is underway to decide whether to permanently remove him from office.

Yoon became the first sitting South Korean president to be arrested last week for declaring a brief period of martial law on December 3.

He was formally detained on Sunday after a court approved a warrant for his arrest, which included having his photo taken, citing concerns that the suspect might destroy evidence.

Angry Yoon supporters stormed the Seoul West District Courthouse early Sunday after the midnight ruling, destroying property and clashing with police, who were at times overpowered by mobs wielding broken barricades and attacking them.

According to Yonhap News Agency, the police plan to arrest 66 people on charges of trespassing, obstructing official duties and assaulting police officers.

Acting Justice Minister Kim Suk-woo told the parliament's Judiciary Committee that other criminals were still being identified and police would also take legal action against them.

Acting President Choi Sang-mok expressed deep regret for the "illegal violence" at the courthouse and urged police to strictly enforce the law to prevent a repeat of what happened on Sunday.

Shortly after the 3 a.m. ruling on Sunday approved Yoon's detention, hundreds of protesters broke through police cordons and entered the court building, with some blasting fire extinguishers at police lines.

In the video footage, some of them can be seen roaming the halls where the judge's office is located, shouting out the name of the judge who approved the arrest warrant.

National Court Chief Executive Quan Daye said at least one judge's office had been forcibly broken into.

Several people involved live-streamed the intrusion on YouTube, with video showing protesters disrupting the courthouse and chanting Yin's name. Some anchors were caught by police during live broadcasts.

Mr Yoon on Monday refused to appear for questioning at the Corruption Investigation Office (CIO), the senior official leading the criminal investigation, after repeatedly refusing to cooperate with the investigation.

His lawyers argued that his arrest on Wednesday and the detention order issued against him were unlawful because they were upheld by a court with the wrong jurisdiction and that the CIO itself had no legal authority to investigate.

The rebellion Yoon may be charged with is one of the few crimes for which South Korea's president does not have immunity and technically carries the death penalty. However, South Korea has not executed anyone in nearly 30 years.

Yoon said through his lawyer that he found the violence in the courtroom on Sunday "shocking and unfortunate" and called on people to express their opinions peacefully. Yin also said in the statement that he understood that many people felt "angry and unfair" and asked the police to take a tolerant stance.