South Korea's parliament introduced a motion on Thursday to impeach President Yoon Suk Yeol over a botched attempt to impose martial law, while the defence minister blamed for advising the move and ordering troops to the parliament resigned.

Lawmakers from the opposition Democratic Party planned to put up a vote in parliament to impeach Yoon at around 7 p.m. local time on Saturday, a party spokesperson told reporters.

The attempt "caused great confusion and fear among our people," Democratic Party lawmaker Kim Seung-won told a session of South Korea's National Assembly held in the early hours of Thursday.

The party needs at least eight of the ruling party's 108 lawmakers to back the bill for it to pass with a two-thirds majority of the 300-seat parliament.

Yoon's ruling People Power Party is divided over the crisis but, with two years left in Yoon's five-year term, said it would oppose impeachment.

Fighting for his political future, Yoon accepted the resignation of Defence Minister Kim Yong-hyun on Thursday and nominated his ambassador to Saudi Arabia, Choi Byung-hyuk, as a replacement, Yoon's office said.

Kim had recommended Yoon declare martial law late on Tuesday, according to a senior military official and the filing to impeach Yoon by opposition members. Kim also ordered the deployment of troops to the parliament, Vice-Defence Minister Kim Seon-ho said, adding he was unaware of the martial law order until Yoon declared it.

"I have fundamentally opposed the mobilization of military forces under martial law and have expressed negative opinions about it," he told a parliament hearing on Thursday, apologizing and taking responsibility for failing to prevent it.

Martial law lasted about six hours, as the National Assembly quickly voted to overrule the president, forcing his cabinet to lift it before daybreak Wednesday.

The declaration of martial law attempted to ban political activity and censor the media in Asia's fourth-largest economy and a key U.S. ally. It sparked outrage in the streets and concern among its international allies.

Japanese Prime Minister Shigeru Ishiba said Japan's "security situation may be fundamentally changed" in light of the instability in Seoul and North Korea's rising military assertiveness.

"What will happen to South Korea? There appears to be a great deal of domestic criticism and opposition," he told parliament on Thursday, adding that Yoon's efforts to improve relations with Tokyo "must never be undermined."

There has been no reaction yet from North Korea to the drama in the South.

U.S. Secretary of State Antony Blinken told Reuters on Wednesday Washington had not been made aware in advance of Yoon's declaration, while his deputy, Kurt Campbell, said Yoon had badly misjudged it.

Yoon had been embraced by leaders in the West as a partner in the U.S.-led effort to unify democracies against growing authoritarianism in China, Russia and elsewhere.

But he caused unease among South Koreans by branding his critics as "communist totalitarian and anti-state forces." In November, he denied wrongdoing in response to influence-peddling allegations against him and his wife and he has taken a hard line against labour unions.

The impeachment follows a night of chaos after Yoon declared martial law and armed troops attempted to force their way into the National Assembly building in Seoul, only to stand back when parliamentary aides sprayed them with fire extinguishers.

The commander of the martial law troops said he had no intention of wielding firearms against the public, and Kim, the vice defence minister, said no live ammunition had been provided to those troops.

"The people and the aides who protected parliament protected us with their bodies. The people won, and it's now time for us to protect the people," the Democratic Party's Kim said.

"We need to immediately suspend the authority of President Yoon. 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."

The martial law crisis rattled global financial markets and South Korea's benchmark KOSPI index. Currency dealers reported suspected state intervention on Wednesday to keep the won stable.

Finance Minister Choi Sang-mok sent an emergency note to global financial chiefs and credit rating agencies late on Wednesday to say the ministry was working to alleviate any adverse impact from political turmoil.

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.