Cho Kuk, leader of the minor opposition Rebuilding Korea Party, said early Wednesday that he believed President Yoon Suk Yeol's declaration of martial law was "illegal," adding that it goes against the law and is a move that warrants the president's removal from office.

"If President Yoon Suk Yeol does not comply with the National Assembly's resolution (to lift the emergency martial law), that would be considered both illegal and a criminal act," Rep. Cho told reporters after the emergency plenary session held hours after the president declared martial law.

"We have got to check whether the procedure of declaring martial law today followed the law and met the conditions for the martial law. I believe neither of those conditions was met," Rep. Cho noted.

The former justice minister also said that the president and Defense Minister Kim Yong-hyun should be punished for declaring martial law, adding that the actions meet all the necessary conditions to be impeached by the National Assembly.