Seoul (AFP) - 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.

Lawmakers quickly voted to oppose the shock declaration but the military backed Yoon, setting up a stand-off with no clear way forward.

The National Assembly was sealed late on Tuesday night and helicopters were seen landing on the roof.

Troops entered the building for a short time, while hundreds of protesters gathered outside parliament chanting: “arrest Yoon Suk Yeol” and faced off with security forces guarding parliament.

“After I saw reports soldiers had gone inside the assembly, I had to come here to protect our democracy,” protester Lee Jin-hwa, 48, told AFP outside parliament.

Some 190 lawmakers managed to get in to the assembly in the early hours of Wednesday, where they unanimously voted in favour of a motion to block the martial law declaration and call for its lifting.

Under the constitution, martial law must be lifted when a majority in parliament demands it.

Yoon gave a range of reasons to justify his announcement – South Korea’s first declaration of martial law in more than 40 years.

“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.

Yoon did not give details of the North’s threats, but the South remains technically at war with nuclear-armed Pyongyang.

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

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.

Army chief General Park An-su took charge as martial law commander and immediately issued a decree banning “all political activities”.

Democratic South Korea is a major ally for the United States in Asia, and the US State Department said it had “grave concern” about the situation.

“We are watching the recent developments in the ROK with grave concern,” Campbell said, referring to South Korea by its official name, the Republic of Korea.

“We have every hope and expectation that any political disputes will be resolved peacefully and in accordance with the rule of law,” he said.

China, a key ally of North Korea, urged its nationals in the South to stay calm and exercise caution, while Britain said it was “closely monitoring developments”.

- ‘Anti-state’ forces -

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, without elaborating further.

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”.

- Budget row -

Vladimir Tikhonov, professor of Korea studies at the University of Oslo, said Yoon’s act was “an attempt to wind the history back”.

“I don’t think South Korea’s civil society can recognise Yoon as a legitimate president any longer,” he told AFP.

Yoon’s People Power Party and the main opposition Democratic Party are bitterly at odds over next year’s budget.

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

The opposition has slashed approximately 4.1 trillion won ($2.8 billion) 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.

The imposition of emergency martial law came after Yoon’s approval rating dropped to 19 percent 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.