His move to impose military rule on December 3 was rescinded barely six hours later after parliament defied troops and police to vote against the decree, but it plunged the country into a constitutional crisis and triggered widespread calls for him to step down for breaking the law.
Opposition parties plan to hold an impeachment vote at 4pm on Saturday (6pm AEDT), with large demonstrations planned ahead of the vote.
South Korea President Yoon Suk-yeol's declaration of martial law caused widespread protests. (AP PHOTO)
Yoon's conservative People Power Party boycotted the first impeachment vote a week earlier, preventing a quorum.
Since then, PPP leader Han Dong-hoon has urged party members to vote for impeachment on Saturday, and at least seven PPP members have said they would vote to impeach.
The opposition parties control 192 of the 300 seats in the single-chamber parliament so they need at least eight PPP votes to reach the two-thirds threshold for impeachment.
If impeached, Yoon would lose authority but remain in office until the Constitutional Court either removes or reinstates him. In the meantime, Prime Minister Han Duck-soo would serve as acting president.
If the court removes Yoon or he resigns, a presidential election must be held within 60 days.
Yoon is separately under criminal investigation for alleged insurrection over the martial law declaration and authorities have banned him from travelling overseas.
He has not signalled a willingness to resign and in a speech on Thursday vowed he would "fight to the end" and defended the martial law decree as necessary to overcome political deadlock and protect the country from domestic politicians who are undermining democracy.
Yoon, president of Asia's fourth-largest economy, hopes political allies will rally to support him, but the fiery remarks appeared to find mixed reception among PPP lawmakers.
