The leader of South Korea's ruling party, Han Dong-hoon, said he was resigning, Yonhap and Reuters reported, quoted by BTA.
Han had called for the impeachment of President Yoon Suk-yeol on Thursday, ahead of a vote in parliament on Saturday. Despite the backlash from within the ruling party, he said he did not regret his decision.
The results of Saturday's vote showed that 12 ruling party lawmakers broke party lines and supported impeachment, Yonhap reported.
South Korea's Constitutional Court will begin hearing today the impeachment motion against President Yoon Suk-yeol over his attempt to impose martial law on December 3, a court spokesman said. All six current judges will attend the first hearing of the impeachment trial, which the opposition-led parliament passed on Saturday.
The court has six months to decide whether to remove Yoon from office or reinstate him.
Yoon and a number of senior officials face potential charges of rebellion under the short-lived martial law.
A joint team of investigators from the police, defense ministry and anti-corruption agency plans to summon Yoon for questioning on Wednesday, Yonhap reported. Yoon failed to appear yesterday in response to a subpoena for questioning as part of a separate investigation by prosecutors.