South Korea has plunged into political crisis after the country’s president made an extraordinary declaration of martial law in a surprise late-night television address.

President Yoon Suk Yeol claimed his sudden move was about protecting South Korea from “shameless pro-North Korean anti-state forces”.

The shocking declaration sparked furious reactions from MPs on all sides of politics and angry backlash from protesters who clashed with troops outside parliament.

Footage showed troops apparently tasked with imposing martial law attempting to enter the assembly building.

Parliamentary aides were seen trying to push the soldiers back by spraying fire extinguishers.

The military said activities by parliament and political parties would be banned, and that media and publishers would be under the control of the martial law command.

After the declaration, 190 of 300 members of parliament quickly convened and unanimously passed a motion demanding the president lift the martial law.

In breaking news on Wednesday morning (6.30am AEDT), the president announced he would honour the parliamentary vote and lift martial law just hours after the declaration.

Protesters outside parliament shouted and clapped after Yoon backed down.

“We won!” they chanted. One demonstrator banged on a drum.

South Korea became a parliamentary democracy in 1989 and has not had military rule since 1979 when dictator Park Chung-hee was assassinated.

A crowd builds outside South Korea’s National Assembly. Photo: AAP

Earlier, National Assembly speaker Woo Won Shik declared that MPs “will protect democracy with the people”.

Woo called for police and military personnel to withdraw from the Assembly’s grounds.

Han Dong-hoon, the chief of the president’s own People Power Party, said the declaration of martial law was “wrong” and that he would “block it” together with the people.

Yoon — whose approval rating in polls has dipped in recent months — has struggled to push his agenda against an opposition-controlled parliament since taking office in 2022.

Yoon did not cite any specific threat from the nuclear-armed North, instead focusing on his domestic political opponents.

He claimed on Tuesday night that opposition parties had taken the parliamentary process hostage.

Yoon vowed to eradicate “shameless pro-North Korean anti-state forces” and said he had no choice but to take the measure to safeguard constitutional order.

“I declare martial law to protect the free Republic of Korea from the threat of North Korean communist forces, to eradicate the despicable pro-North Korean anti-state forces that are plundering the freedom and happiness of our people and to protect the free constitutional order,” Yoon said.

Shortly after Yoon made his announcement, people began gathering outside the parliament building, some of them shouting: “Withdraw emergency martial law!”

Troops mobilise after the martial law declaration. Photo: AAP

US Deputy Secretary of State Kurt Campbell said the United States 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.

About 28,500 US troops are stationed in South Korea.

Korea’s currency, the won, was down sharply against the US dollar.

A central bank official said it was preparing measures to stabilise the market if needed.

Yoon said he had no choice but to resort to such a measure in order to safeguard free and constitutional order, saying opposition parties have taken hostage of the parliamentary process to throw the country into a crisis.

South Korean President Yoon Suk Yeol declared martial law. Photo: AAP

Yonhap reported that the entrance to the parliament building was blocked.

“Tanks, armoured personnel carriers and soldiers with guns and knives will rule the country,” Lee Jae-myung, leader of the opposition Democratic Party, said in a live-stream online.

“The economy of the Republic of Korea will collapse irretrievably. My fellow citizens, please come to the National Assembly.”

Yoon cited a motion by the Democratic Party, which has a majority in parliament, this week to impeach some of the country’s top prosecutors and its rejection of a government budget proposal.

South Korea’s ministers on Monday protested the move by the opposition last week to slash more than 4 trillion won ($4.3 trillion) from the government’s budget proposal.

Yoon said that action undermines the essential functioning of government administration.

-with AP and Yonhap