SEOUL - South Korea's acting President Choi Sang-mok on Sunday ordered a stern investigation into the violence of the supporters of impeached President Yoon Suk-yeol at a Seoul court that granted a warrant for Yoon's extended detention.

Choi said in a statement that the government expresses strong regret over illegal violence, which is unimaginable in a democratic society.

The acting president ordered the police to sternly investigate the incident, vowing to hold violators legally responsible.

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He instructed relevant authorities to tighten security around relevant facilities and thoroughly manage protest rallies against Yoon's extended detention.

The Seoul Western District Court issued the warrant early Sunday to detain the already arrested Yoon for up to 20 days over his martial law imposition, making Yoon the country's first sitting president to be formally arrested.

Following the decision, some of the enraged supporters of Yoon stormed the court building by climbing over walls and breaking windows.

They vandalized facilities inside the court, threw trash and other objects, and threatened police officers. The number of violators who were apprehended at the scene was 86, according to multiple media outlets.

The country’s top court also denounced the violence of Yoon supporters as a grave "challenge to rule of law."

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Chun Dae-yup, the national court administration chief under the direction of the Supreme Court, said in a statement that the top court expresses serious concern and strong regret over the intrusion of some protesters into the Seoul Western District Court.

Chun stressed that it was "the general denial of and the grave challenge to rule of law," which should never happen and be tolerated, adding that it should be followed by doing thorough fact-checking and holding violators strictly accountable.

The impeachment motion against Yoon was passed in the National Assembly on Dec 14 last year and was delivered to the constitutional court to deliberate it for up to 180 days, during which Yoon's presidential power is suspended.

Yoon, who was named by investigative agencies as a suspected ringleader on an insurrection charge, declared martial law on the night of Dec 3 last year but it was revoked by the National Assembly hours later.