South Korean President Yoon Suk Yeol has accepted the resignation of Defence Minister Kim Yong-hyun and nominated the ambassador to Saudi Arabia, Choi Byung-hyuk, as new defence minister, the presidential office said on Thursday.

Kim, who offered his resignation on Wednesday, was seen as a central figure in Yoon's declaration of martial law on Tuesday. A senior military official and filings to impeach Yoon by opposition members said Kim made the recommendation to Yoon.

Yoon's chief of staff, Chung Jin-suk, called Choi "a person of principle who carries out his duties with dedication and adheres to regulations."

The nomination is the first official move by Yoon after he announced the withdrawal of the martial law declaration in the early hours of Wednesday, amid a storm of political turmoil and diplomatic fallout.

Parliament introduced a motion early on Thursday to impeach Yoon over the botched attempt to impose martial law, but his party vowed to oppose the move, throwing the process into doubt.

WATCH | Yoon faces impeachment:

South Korean opposition parties seek to impeach President Yoon over martial law order

Thousands of people marched in Seoul on Wednesday demanding President Yoon Suk Yeol resign over his short-lived declaration of martial law in South Korea. Lawmakers are expected to vote on the impeachment of Yoon on Friday or Saturday.

The main opposition Democratic Party has called Yoon's martial law attempt a treasonous act, and its lawmakers could lead a vote for the bill as early as Friday.

U.S. officials said they were caught completely off guard by Yoon's martial law declaration, which U.S. Deputy Secretary of State Kurt Campbell said was "badly misjudged."

Yoon's martial law declaration, the first of its kind in decades, harkened back to South Korea's past military-backed governments when authorities occasionally proclaimed martial law and other decrees that allowed them to station soldiers, tanks and armoured vehicles on streets or at public places such as schools to prevent anti-government demonstrations.

Until Tuesday night and Wednesday morning, such scenes of military intervention had not been seen since South Korea achieved a democracy in the late 1980s.