South Korean MPs begin impeachment process against President Yoon Suk Yeol as his party demands he leave their ranks

South Korean MPs have formally moved to impeach the country's president, accusing him of declaring martial law to prevent criminal investigations into himself and his family.

Opposition parties introduced an impeachment motion and a complaint of "insurrection" against President Yoon Suk Yeol to the nation's parliament just after midnight on Thursday morning.

Mr Yoon's surprise declaration of martial law on Tuesday was swiftly overturned by MPs, but not before chaotic scenes in and around the parliament, the National Assembly in Seoul.

The impeachment motion alleges Mr Yoon "gravely and extensively violated the constitution and the law" and accuses him of imposing martial law "with the unconstitutional and illegal intent to evade imminent investigations … into alleged illegal acts involving himself and his family".

The main opposition Democratic Party has also filed an "insurrection" complaint against the president, some of his ministers and top military and police officials. The offence can carry a penalty of life imprisonment or even death.

The opposition holds a large majority in the 300-member parliament. Eight members of the president's party need to back the impeachment motion to give it the 200 votes required for it to pass.

To officially remove the president from office, it also has to be approved by South Korea's constitutional court.

The leader of the ruling People Power Party, Han Dong-hoon, said it would block the opposition-led impeachment effort but also demanded the president resign from the party.

"We will make efforts to stop this impeachment [motion] from passing," Mr Han told party members during a meeting that was broadcast live.

He then told reporters he had "demanded the president's resignation from the party", which was "not trying to defend the president's unconstitutional martial law".

In the party meeting, floor leader Choo Kyung-ho said all 108 of its politicians would "stay united" to defeat the impeachment motion. The opposition needs eight ruling party politicians to vote with them for the impeachment bill to pass.

The convening of a late-night parliamentary session was a de facto deadline for the president to resign, which he didn't do.

Mr Yoon announced the martial law declaration during a late-night television address, complaining of pro-North Korea "anti-state forces" at work in the country and claiming opposition parties were holding the parliament hostage.

Minister of defence resigns

Defence Minister Kim Yong-hyun, who according to Korean media personally suggested the martial law idea to the president, offered his resignation on Wednesday evening, saying he took all responsibility for the stunning turn of events.

The presidential office confirmed it had accepted Mr Kim's resignation and that Choi Byung-hyuk, South Korea's current ambassador to Saudi Arabia and a former army general, had been nominated as the new defence minister.

The president's chief of staff called Mr Choi "a person of principle who carries out his duties with dedication and adheres to regulations."

The nomination is Mr Yoon's first official move after he announced the withdrawal of the martial law declaration in the early hours of Wednesday.

Despite freezing temperatures, thousands lined the streets and protested outside the South Korean National Assembly on Wednesday night.

People were outside the National Assembly all day and some stayed until the late hours of the evening, for a second day in a row.

A group of university students were there late into the night, and told the ABC they wanted to see the president impeached as soon as possible.

Journalism student Ahn Jung Eun, 24, said the president's actions were "completely unexpected and unimaginable".

"That shows he should be punished and impeached.

"I saw last night even after martial law was declared, people were not afraid to come here and thousands gathered here to show how great South Korean people are and that we would never surrender."

Many of those who protested outside the National Assembly expressed their exasperation with the president and the events of the last 24 hours.

One man told the crowd he was a farmer who was visiting from the countryside when he heard the news, and had stayed outside the assembly all night.

"I couldn't bear it anymore," said Cho Young Mun, who'd travelled from Incheon, about an hour away.

"For the last two years, he was so incompetent in handling foreign affairs, national affairs, and also the everyday lives of people.

"I can't understand why he did it in this situation, where economy is in turmoil and national defence is in a mess and everything is in the chaos, why he decided to do that irresponsibly."

Park Min Kyun, 35, said he couldn't sleep after hearing the news martial law had been declared on Tuesday night.

"It shows that our democracy is not as strong as we thought," he said.

"And I think this is the time for us to review our history and make sure that this kind of thing will never happen again."

If Mr Yoon quits or is impeached, Prime Minister Han Duck-soo will step in until a replacement is elected, which has to happen within 60 days.

Mr Han is also a conservative and was appointed prime minister by Mr Yoon.

ABC/wires