By Joyce Lee and Hyunjoo Jin

SEOUL (Reuters) -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. (1000 GMT) on Saturday, a party spokesperson told reporters.

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

"The Yoon Suk Yeol regime's declaration of emergency martial law 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 Democratic Party needs at least eight of the 108 ruling-party lawmakers to back the bill for it to pass with a two-thirds majority of the 300-seat parliament.

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 mobilisation of military forces under martial law and have expressed negative opinions about it," he told a parliament hearing on Thursday, apologising and taking responsibility for failing to prevent it.

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.

Secretary of State Antony Blinken told Reuters on Wednesday the United States 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.