South Korean President Yoon Suk Yeol has declared emergency martial law in a surprise late-night televised address to the nation on Tuesday.

President Yoon made the shock declaration in a live address on YTN television - where he said he had to do it to fend off opposition parties which had taken hold of the country's parliamentary process.

He added that the entrance to the National Assembly, the seat of government in South Korea, had been blocked - with lawmakers unable to enter.

Meanwhile, South Korea's military has said activities by parliament and political parties would be banned, and that media and publishers would be under the control of Yoon's martial law.

The White House has said the US was "monitoring the situation closely" as images flooded in from capital city Seoul on Tuesday afternoon UK time, showing clashes between police and large crowds on the South Korean parliament building.

"To safeguard a liberal South Korea from the threats posed by North Korea's communist forces and to eliminate anti-state elements... I hereby declare emergency martial law," Yoon said in his address.

The president emphasised that the measure was necessary to protect the country's constitutional order. He stated his intention to "rebuild a free and democratic country" through the implementation of martial law.

Yoon claimed he had no choice but to resort to such measures to safeguard what he called "the free Republic of Korea."

The specific measures to be implemented under the martial law were not immediately detailed in his address.

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In his address, Yoon accused opposition parties of taking control of parliament and paralysing the government through anti-state activities. The president specifically targeted the Democratic Party, which holds a majority in parliament.

"Tanks, armored personnel carriers, and soldiers with guns and knives will rule the country," Lee Jae-myung, leader of the opposition Democratic Party, which has the majority in parliament, said in a livestream 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 country's opposition 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 opposition DP last week to slash more than four trillion won from the government's budget proposal.

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