Yoon said opposition parties had taken the parliamentary process hostage. He 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.
Yonhap news agency cited the military as saying activities by parliament and political parties would be banned, and that media and publishers would be under the control of the martial law command.
Yoon did not cite any specific threat from the nuclear-armed North, instead focusing on his domestic political opponents. It is the first time since 1980 that martial law has been declared in South Korea.
South Korea has had a series of authoritarian leaders early in its history but has been considered democratic since the 1980s.
The Korean won was down sharply against the U.S. dollar. A central bank official said it was preparing measures to stabilise the market if needed. Finance Minister Choi Sang-mok has convened an emergency meeting among top economic officials, his spokesman said in a text message.
Yoon s predecessor, Moon Jae-in of the Democratic Party, said in a post on X that the country s democracy is in crisis. "I hope that the National Assembly will act quickly to protect our democracy from crumbling," he wrote in a post. "I ask the people to join forces to protect and save democracy and to help the National Assembly function normally."
The United States is in contact with the South Korean government and is monitoring the situation closely, a White House spokesperson said.
Some 28,500 U.S. troops are stationed in South Korea to guard against the North. A spokesman for the U.S. military command did not answer repeated phone calls.
Japan s prime minister s office could not immediately be reached for a comment outside of business hours.
YOON SLAMS PARLIAMENT
"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.
Yoon did not immediately specify who constituted the pro-North Korean anti-state forces. But he has cited such forces in the past as hindering his agenda and undermining the country.
He did not say in the address what specific measures will be taken. Yonhap reported that the entrance to the parliament building was blocked.
"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.
"Our parliament has become a den for criminals. It has paralyzed the administrative and legal systems by ramming through (opposition-driven) legislations and is attempting to overthrow our democratic system," Yoon said.
South Korea s ministers on Monday protested the move by the opposition DP last week to slash more than 4 trillion won from the government s budget proposal. Yoon said that action undermines the essential functioning of government administration.
South Korea s opposition parties held a rally on Saturday calling for Yoon to accept a special prosecutor investigation on allegations of fraud against the first lady, the latest such protests by the party and civic groups in recent weeks against Yoon.
In October, North Korean propaganda leaflets apparently carried by balloons were found scattered on the streets of Seoul, including some making personal attacks on Yoon and the first lady.
