Yoon Suk Yeol attends the fourth impeachment trial hearing at the Constitutional Court in Seoul, Thursday. Yonhap

President Yoon Suk Yeol was indicted Sunday on charges of leading an insurrection tied to last month's martial law declaration fiasco, becoming the first sitting Korean president to be indicted with detention.

Prosecutors indicted him just a day before Yoon's detention period was set to end, Monday, following his provisional arrest by the Corruption Investigation Office for High-ranking Officials (CIO), Jan. 15. Yoon was formally arrested, Jan. 19.

They had requested an extension of the detention period, citing the need to continue questioning him as the president has mostly refused to cooperate with the investigation. However, the Seoul Central District Court rejected the request twice over the weekend. By law, a suspect must be released if not indicted within the detention period.

Prosecutors have indicted Yoon solely on the charge of leading an insurrection citing "concerns that the suspect may destroy evidence." Yoon had also faced allegations of abuse of power, but these charges were dropped. Under the law, a president is immune from prosecution while in office, except in cases of insurrection.

Earlier on Sunday, Prosecutor General Shim Woo-jung convened a meeting with senior prosecutors from across the country to discuss the next steps in Yoon's case.

The CIO, which had led the investigation into Yoon, transferred the case to the prosecution Thursday after the anti-corruption agency repeatedly failed in its attempts to question the detained president. The prosecution holds the sole authority to indict the president.

The rival political parties showed mixed reactions to Yoon's indictment.

"Prosecutors will not be able to avoid legal and political responsibility for their ‘appalling and sloppy' indictment of the sitting president," Rep. Shin Dong-wook, spokesperson of the ruling People Power Party, said.

On the other hand, the opposition said condemnation of the "insurrection leader" begins now.

"Yoon must no longer mock judges in the sacred court with sophistry, lies and self-denial. Attempts to incite far-right supporters with baseless allegations and delusions must also stop," Rep. Han Min-soo, spokesperson of the main opposition Democratic Party of Korea, said.

Yoon's legal team criticized the prosecution's actions and pledged to expose any unlawful conduct in the investigation.

"The prosecution has made a grave mistake, reducing itself to being the indictment arm of the CIO, and a tool of political interests. Today's indictment of the president has left a lasting blemish on the prosecution's legacy," the team said in a statement.

"We will relentlessly expose the illegalities in the investigation and unveil the truth behind the wrongful claim that the presidential emergency powers constitute an act of insurrection."