If Yoon is impeached, he’ll be stripped of his powers until the Constitutional Court can rule on his fate. Prime Minister Han Duck-soo, the No. 2 in the South Korean government structure, would take over his presidential responsibilities.

Yoon’s martial law declaration harked back to the era of military-backed governments between 1948 and 1987, when authorities occasionally proclaimed martial law and other decrees that allowed them to station combat soldiers, tanks and armoured vehicles on streets or at public places such as schools to prevent anti-government demonstrations. Such scenes had not been seen since South Korea achieved a genuine democracy in the late 1980s.

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After Yoon’s declaration, troops carrying full battle gear, including assault rifles, tried to keep protesters away from the National Assembly as military Blackhawk helicopters flew overhead and landed nearby. One soldier pointed his assault rifle at a woman protesting against martial law outside the building.

It wasn’t clear how the 190 MPs were able to enter a parliamentary hall to vote down Yoon’s decree. Some reportedly climbed over walls, and while troops and police officers blocked some from entering they didn’t aggressively restrain or use force against others.

No major violence has been reported. The troops and police personnel were later seen leaving the grounds of the National Assembly after the parliamentary vote. National Assembly Speaker Woo Won-shik said: “Even with our unfortunate memories of military coups, our citizens have surely observed the events of today and saw the maturity of our military.”

Under South Korea’s constitution, the president can declare martial law during “wartime, war-like situations or other comparable national emergency states” that require the use of military force to restrict the freedom of press, assembly and other rights to maintain order. Many observers question whether South Korea is currently in such a state.

The constitution also states that the president must oblige when the National Assembly demands the lifting of martial law with a majority vote.

US Deputy Secretary of State Kurt Campbell said Washington was watching events in South Korea with “grave concern” and hoped that any political disputes would be resolved peacefully and in accordance with the rule of law. A spokesperson for the US National Security Council said US President Joe Biden’s administration was not notified in advance of the martial law announcement and was in contact with the South Korean government.

Pentagon spokesman Major General Pat Ryder said there was no effect on the more than 27,000 US service members based in South Korea.

A spokesperson for Foreign Minister Penny Wong said Australia was concerned by the political situation.

“As a close partner and friend of the Republic of Korea, we hope for a democratic and peaceful resolution for the Korean people,” the spokesperson said.

“Australians in Korea should monitor Smartraveller for updates to our travel advice.”

South Korea’s currency, the won, fell sharply against the US dollar, and a central bank official said it was preparing measures to stabilise the market if needed.

South Korea’s finance ministry said on Wednesday it was ready to deploy “unlimited” liquidity into financial markets. The announcement came after Finance Minister Choi Sang-mok and Bank of Korea governor Rhee Chang-yong held emergency talks overnight, and the central bank board met to approve rescue measures for the local credit market.

In Seoul, there was little sign of upheavel. Tourist Stephen Rowan, from Brisbane, who was touring Gyeongbokgung Palace, said he was not concerned at all.

“But then again, I don’t understand too much about the political status in Korea,” he said. “But I hear they are now calling for the current president’s resignation, so ... apparently there’s going to be a lot of demonstrations ... I would have been concerned if martial law had stayed enforced.”

Yoon’s government has been embroiled in an impasse with the Democratic Party over next year’s budget bill and a Democratic Party-led attempt to impeach three top prosecutors.

During his televised announcement, Yoon described the opposition as “shameless pro-North Korean anti-state forces who are plundering the freedom and happiness of our citizens”. He did not elaborate. North Korea had no immediate comments.

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Natalia Slavney, research analyst at the Stimson Centre’s 38 North website that focuses on Korean affairs, said Yoon’s imposition of martial law was “a serious backslide of democracy” that followed a “worrying trend of abuse” since he took office in 2022.

South Korea “has a robust history of political pluralism and is no stranger to mass protests and swift impeachments”, Slavney said, citing the example of former president Park Geun-hye, who was ousted from office and imprisoned for bribery and other crimes in 2017. She was later pardoned.

Opposition parties held a rally last Saturday calling for Yoon to accept a special prosecutor’s investigation into allegations of fraud against the first lady, Kim Keon-hee, the latest such protests by the opposition and civic groups.