Updated 9pm

South Korea's President Yoon Suk Yeol said Wednesday martial law would be lifted and troops withdrawn, hours after he made the shock declaration.

"Just a moment ago, there was a demand from the National Assembly to lift the state of emergency, and we have withdrawn the military that was deployed for martial law operations," Yoon said in a televised address.

"We will accept the National Assembly's request and lift the martial law through the Cabinet meeting." 

Celebrations followed the announcement, according to news agency AFP.

In a dramatic, emergency television address to the nation, Yoon late on Tuesday night announced that he was imposing martial law, accusing the opposition of paralysing the government with "anti-state activities".

However, 190 lawmakers managed to get in to the assembly in the early hours of Wednesday, where they unanimously voted 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 his earlier 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.

Assembly closed

Yoon, a former prosecutor, had 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 president went on to label the opposition, which holds a majority in the 300-member parliament, as "anti-state forces intent on overthrowing the regime".

Police struggle with people trying to enter the National Assembly in Seoul. Photo: JUNG YEON-JE / AFP.

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 other than the martial law in place. 

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".

With martial law imposed, all military units in the South, which remains technically at war with the nuclear-armed North, were ordered to strengthen their emergency alert and readiness postures, Yonhap news agency reported.

The entrance to the National Assembly was sealed, and MPs had been barred from entering the building, according to Yonhap.

The imposition of emergency martial law came as Yoon's approval rating dropped to 19 per cent 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.

US 'grave concern'

The United States had expressed "grave concern" over South Korea after President Yoon Suk Yeol declared martial law Tuesday, with Deputy Secretary of State Kurt Campbell calling for a resolution that follows "rule of law".

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

"I do want to underscore that our alliance with the ROK is ironclad, and we stand by Korea in their time of uncertainty."

Yoon has been a close ally of the United States, with US President Joe Biden welcoming him on a state visit last year and South Korea in March hosting a global democracy summit, a signature idea of the outgoing US administration.

Germany had declared its "great concern" for the situation in the country, while the UK had said it was "deeply concerned".