Seoul, South Korea - South Korea's President Yoon Suk Yeol on Tuesday declared martial law, accusing the opposition of being "anti-state forces" and saying he was acting to protect the country from "threats" posed by the North.

The National Assembly was sealed late on Tuesday night and helicopters were seen landing on the roof, as army chief General Park An-su took charge as martial law commander and immediately issued a decree banning "all political activities."

Hundreds of protesters gathered outside parliament demanding to be let inside as police guarded the building and special forces

South Korea is a major ally for the US in Asia, and a spokesperson for the National Security Council said Washington was "monitoring the situation closely."

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"To safeguard a liberal South Korea from the threats posed by North Korea's communist forces and to eliminate anti-state elements plundering people's freedom and happiness, I hereby declare emergency martial law," Yoon said in a live televised address to the nation, without providing any evidence of any immediate North Korean threats.

"With no regard for the livelihoods of the people, the opposition party has paralyzed governance solely for the sake of impeachments, special investigations, and shielding their leader from justice," the 63-year-old conservative politician raged.

"Our National Assembly has become a haven for criminals, a den of legislative dictatorship that seeks to paralyze the judicial and administrative systems and overturn our liberal democratic order."

In response, South Korea's National Assembly immediately passed a resolution rejecting the imposition of martial law.

With martial law imposed, all military units in the South have been ordered to strengthen their emergency alert and readiness postures, Yonhap news agency reported.

The US expressed alarm at the sudden escalation, with Deputy Secretary of State Kurt Campbell saying Washington was monitoring developments with "grave concern."

Meanwhile, China – a key ally of North Korea – urged its nationals in the South to stay calm and exercise caution.

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The decree by martial law commander Park also banned "actions that deny or seek to overthrow the liberal democratic system, including the spread of fake news, public opinion manipulation, and false propaganda".

The president labelled the opposition, which holds a majority in the 300-member parliament, as "anti-state forces intent on overthrowing the regime."

Yoon described the imposition of martial law as "inevitable to guarantee the continuity of a liberal South Korea," adding that it would not impact the country's foreign policy.

"I will restore the country to normalcy by getting rid of anti-state forces as soon as possible," he said.

He described the current situation as South Korea "on the verge of collapse, with the National Assembly acting as a monster intent on bringing down liberal democracy".

Yoon's stunning announcement – South Korea's first declaration of martial law in more than 40 years – came as his party and the opposition were bickering over the budget.

Opposition MPs last week approved a significantly downsized budget plan through a parliamentary committee.

Some 4.1 trillion won ($2.8 billion) were slashed from Yoon's proposed 677 trillion won budget plan – cutting the government's reserve fund and activity budgets for Yoon's office, the prosecution, police, and the state audit agency.

Yoon accused opposition lawmakers of cutting "all key budgets essential to the nation's core functions, such as combatting drug crimes and maintaining public security... turning the country into a drug haven and a state of public safety chaos."

The shock development comes as Yoon's approval rating dropped to 19% in the latest Gallup poll last week, with many expressing dissatisfaction over his handling of the economy and controversies involving his wife, Kim Keon Hee.