SEOUL — Hundreds of supporters of South Korea's arrested President Yoon Suk-yeol stormed a court building on Sunday after his detention was extended, smashing windows and breaking inside, an attack the country's acting leader called "unimaginable".

Yoon on Wednesday became the first sitting South Korean president to be arrested as he faces allegations of insurrection related to his short-lived Dec 3 declaration of martial law that has plunged the country into political turmoil.

Shortly after the Seoul Western District Court announced the extension of Yoon's detention around 3 am on Sunday, his supporters swarmed the building, overwhelming the riot police trying to keep them at bay.

Protesters blasted fire extinguishers at lines of police guarding the front entrance, then flooded inside, destroying office equipment, fittings and furniture, footage showed.

Police restored order a few hours later, saying they had arrested 46 protesters and vowing to track down the others involved.

"The government expresses strong regret over the illegal violence ... which is unimaginable in a democratic society," acting President Choi Sang-mok said in a statement, adding he ordered the police to sternly investigate the incident.

Nine police officers were injured in the chaos, Yonhap reported.

With Yoon refusing to be questioned, investigators facing a deadline on detaining the president asked the court on Friday to extend his custody.

After a five-hour hearing on Saturday, a judge granted a new warrant extending Yoon's detention for up to 20 days, because of "concern that the suspect may destroy evidence".

On Sunday, Yoon's People Power Party called the court's decision to extend his detention a "great pity".

The main opposition Democratic Party said the decision was a "cornerstone" for rebuilding order.

Agencies via Xinhua