South Korea's parliament has formally introduced a motion to impeach President Yoon Suk Yeol over a botched attempt to impose martial law, but his party vowed to oppose the move, throwing the process into doubt.
Politicians could vote for the bill as early as Friday, but Mr Yoon's ruling People Power Party said they would oppose it.
The opposition Democratic Party, which has a majority in parliament, needs at least eight ruling party politicians to back the bill in order for it to pass.
"The Yoon Suk Yeol regime's declaration of emergency martial law caused great confusion and fear among our people," Democratic Party politician Kim Seung-won told a late night session of South Korea's National Assembly.
None of the 108 ruling party politicians were present for the introduction of the motion.
"We need to immediately suspend the authority of President Yoon," Mr Kim said.
"He has committed an indelible, historic, crime against the people, whose anxiety needs to be soothed so that they can return to their daily lives," he added.
If the impeachment motion is passed and upheld by the constitutional court, Mr Yoon would be the second South Korean president to have been impeached since massive candlelit protests against an influence-peddling scandal led to the removal of former president Park Geun-hye in 2017.
Mr Yoon's declaration of martial law late yesterday attempted to ban political activity and censor the media in South Korea, which has Asia's fourth largest economy and is a key US ally.
The shock move divided Mr Yoon's ministers and his ruling People Power Party and unleashed six hours of political chaos.
Armed troops forced their way into the National Assembly building in Seoul but stood back when parliamentary aides sprayed them with fire extinguishers.
Politicians rejected the martial law decision while protesters scuffled with police outside.
Demonstrators held candlelit vigils in Seoul and called for Mr Yoon's resignation.
Meanwhile, Mr Yoon has accepted the resignation of Defence Minister Kim Yong-hyun and nominated ambassador to Saudi Arabia, Choi Byung-hyuk, as new defence minister, Yonhap news reported.
Mr Yoon's office confirmed the nomination of Mr Choi, a former army general, for the office.
Mr Kim has been reported as having advised Mr Yoon to declare martial law yesterday
The impeachment motion against Mr Yoon was reported to a plenary session that opened shortly after midnight (3pm Irish time) and paved the way for an impeachment vote to be held in the following 24 to 72 hours.
Opposition parties need a two-thirds majority to pass the bill, and support from about eight members from Yoon's own party to secure the necessary 200 votes to impeach him.
If the bill passes, South Korea's Constitutional Court will then decide whether to uphold the motion - a process that could take up to 180 days.
If Mr Yoon were to be suspended from exercising power due to parliament passing the bill, Prime Minister Han Duck-soo would fill in as leader.
If the embattled president resigned or was removed from office, a new election would be held within 60 days.
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In recent weeks, Mr Yoon and his People Power Party have been bitterly at odds with the opposition over next year's budget.
His approval rating dropped to 19% in the latest Gallup poll last week, with voters angry at the state of the economy as well as controversies involving his wife, Kim Keon Hee.
Mr Yoon's action took allies by surprise, with the United States, which has nearly 30,000 troops in the country to protect it from the nuclear-armed North, saying it had no prior notice and voicing relief at his reversal.
South Korea's neighbour China declined to comment on what it said were Seoul's "internal affairs", but did urge Seoul to take measures to ensure the safety of Chinese citizens.
