A photo of a letter, written by impeached President Yoon Suk Yeol, is posted on his Facebook hours after his arrest over his short-lived martial law declaration, Jan. 15. Yonhap

Impeached President Yoon Suk Yeol on Wednesday claimed that "martial law is not a crime," defending his short-lived imposition of martial law as an act of governance in a handwritten letter following his detention.

Yoon reiterated his earlier claims in a Facebook post, hours after he was detained by investigators for questioning over insurrection charges related to his martial law decree on Dec. 3.

"Martial law is not a crime. Martial law is an exercise of presidential authority to overcome a national crisis," Yoon wrote, posting a photo of his handwritten letter.

Yoon, whose duties have been suspended since he was impeached by the National Assembly on Dec. 14, argued his impeachment was based on the narrative equating the martial law declaration with insurrection, saying it is "truly absurd."

He also criticized the opposition's recent withdrawal of the insurrection charge in the impeachment proceedings, calling it "a fraudulent impeachment."

The letter was shared while Yoon declined to testify during questioning by the Corruption Investigation Office for High-ranking Officials. (Yonhap)