Seoul (AFP): South Korean President Yoon Suk Yeol was still clinging to power Thursday, with his party announcing they will oppose an opposition impeachment motion put forward after his stunning but brief imposition of martial law.

Yoon suspended civilian rule late Tuesday and deployed troops and helicopters to parliament only for lawmakers to vote down the measure and force him into a U-turn in a night of protests and drama.

Seoul's allies were alarmed -- Washington said it found out via television -- and on Wednesday, the opposition filed an impeachment motion saying Yoon "gravely violated the constitution and the law".

The opposition aims to bring the bill to a vote Saturday, Yonhap reported.

They hold a large majority in the 300-member legislature and need only a handful of defections from Yoon's People Power Party (PPP) to secure the two-thirds majority needed to pass.

But on Thursday, the PPP leader said that while he had asked Yoon to leave the party, he would block the impeachment motion.

Han Dong-hoon told reporters his party was "not trying to defend the president's unconstitutional martial law".

"All 108 lawmakers of the People Power Party will stay united to reject the president's impeachment," the party's floor leader Choo Kyung-ho said.

If the motion passes, Yoon will be suspended pending a verdict by the Constitutional Court. If the judges give the nod, Yoon will be impeached and new elections must happen within 60 days.

Bad memories
Yoon, who has lurched from crisis to crisis since taking office in 2022, has not been seen in public since his televised address in the early hours of Wednesday.

On Thursday, his office said that Defence Minister Kim Yong-hyun had resigned, but other key allies including Interior Minister Lee Sang-min remain in office.

Yoon's martial law declaration was the first in more than four decades in South Korea and brought back painful memories of the country's turbulent past.

The move was 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," Yoon said.

A six-point decree from the army chief banned political activities and parties, "false propaganda", strikes and "gatherings that incite social unrest".

Security forces sealed the National Assembly, helicopters landed on the roof and almost 300 soldiers tried to lock down the building, seemingly to prevent lawmakers from entering.

But as parliamentary staffers blocked the soldiers with sofas and fire extinguishers, enough MPs -- some leaping over barriers -- got inside and voted down Yoon's move.

This brought cheers from the hundreds of protesters braving bitter temperatures outside, many waving national flags and chanting for Yoon to be arrested.

'Unforgivable'
Lawmakers formally presented the impeachment motion in the early hours of Thursday, saying Yoon's decision to impose martial law was intended to "evade imminent investigations... into alleged illegal acts involving himself and his family".

"This is an unforgivable crime -- one that cannot, should not, and will not be pardoned," MP Kim Seung-won said.

The main opposition Democratic Party has also filed a complaint of "insurrection" against the president, ministers and top military and police officials -- which can carry a penalty of life imprisonment or even death.

In a show of public anger, thousands of protesters converged around Yoon's office in Seoul late Wednesday after staging a rally in Gwanghwamun Square, demanding his resignation.

'Robust and resilient'
The United States has around 30,000 troops stationed in North Korea to help protect its ally against the nuclear-armed North.

US National Security Advisor Jake Sullivan said Wednesday that Yoon's imposition of martial law "raised deep concern for us" while praising the National Assembly for operating "according to constitutional processes and procedures" to rescind it.

"South Korea's democracy is robust and resilient, and we're going to continue to speak out publicly and engage privately with South Korean counterparts to reinforce the importance of that continuing," Sullivan said.

China, a key ally of nuclear-armed North Korea, urged its citizens to exercise caution, while Russia -- increasingly close to Pyongyang -- called the situation "alarming".

Japan, whose historically prickly relations with Seoul have thawed under Yoon, said it was following the situation with "exceptional and serious concerns".