Meanwhile police arrested the defence minister in charge of the martial law operation, and the interior minister resigned. Both they and Yoon are being investigated for alleged insurrection.

Yoon averted impeachment late Saturday as huge crowds braved freezing temperatures in another night of protests outside parliament to demand the president's ouster.

Opposition parties proposed the impeachment motion, which needed 200 votes in the 300-member parliament to pass, but a near-total boycott by Yoon's People Power Party (PPP) doomed it to failure.

Lee Jae-myung, leader of the main opposition Democratic Party (DP), said Sunday that they will try again on December 14.

"Yoon, the principal culprit behind the insurrection and military coup that destroyed South Korea's constitutional order, must either resign immediately or be impeached without delay," Lee told reporters.

"On December 14, our Democratic Party will impeach Yoon in the name of the people."

- 'Soft coup' -

In exchange for blocking his removal from office, Yoon's People Power Party (PPP) said that it had "effectively obtained (Yoon's) promise to step down".

"Even before the president steps down, he will not interfere in state affairs, including foreign affairs," PPP leader Han Dong-hoon said Sunday after a meeting with Prime Minister Han Duck-soo.

This will "minimise the confusion to South Korea and its people, stably resolve the political situation and recover liberal democracy," Han told reporters.

But Lee and the National Assembly speaker Woo Won-shik, both from the opposition Democratic Party (DP), on Sunday called the arrangement illegal.

"For the prime minister and the ruling party to jointly exercise presidential authority, which no one has granted them, without participating in constitutional processes to address unconstitutional martial law, is a clear violation of the Constitution," Woo said.

"The power of the president is not the personal property of President Yoon Suk Yeol," said Lee. "Isn't this another coup that destroys the constitutional order?"

Kim Hae-won, a constitutional law professor at Pusan National University Law School, called it a an "unconstitutional soft coup."

"In reality, a political party is merely a private political entity, and handing over the president's functions to an entity that is neither a constitutional institution nor a state body seems like an action that disrupts the state's rights," Kim told AFP.

- Sorry -

On Saturday before the vote, Yoon, 63, reappeared for the first time in three days and apologised for the "anxiety and inconvenience" caused by his declaration of martial law.

But he stopped short of stepping down, saying he would leave it to his party to decide his fate.

Massive crowds -- police said there were 150,000 people, organisers one million -- gathered outside parliament to pressure lawmakers to oust the president.

Many wore elaborate outfits, carrying home-made flags and waving colourful glow sticks and LED candles as K-pop tunes blasted from speakers.

"Even though we didn't get the outcome we wanted today, I am neither discouraged nor disappointed because we will get it eventually," said protester Jo Ah-gyeong, 30, after the impeachment vote.

"I'll keep coming here until we get it," she told AFP.

- Insurrection -

Regardless of the political situation, police are investigating Yoon and others for alleged insurrection over the extraordinary events of Tuesday night.

Early Sunday police arrested Kim Yong-hyun, who quit as defence minister on Wednesday and was slapped with a travel ban, reports said.

Interior Minister Lee Sang-min on Sunday tendered his resignation which was accepted, Yoon's office said.

Declaring martial law late Tuesday, Yoon said it would safeguard South Korea "from the threats posed by North Korea's communist forces and eliminate anti-state elements plundering people's freedom and happiness".

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

But as parliamentary staffers blocked the soldiers with sofas and fire extinguishers, enough MPs got inside -- many climbed walls to enter -- and voted down Yoon's move.

The episode brought back painful memories of South Korea's autocratic past and blindsided its allies, with the US administration only finding out via television.

"This is a country we've spent our entire lives building," Shin Jae-hyung, 66, who suffered arrest and torture in the 1970s and 80s as he battled successive military-led regimes, told AFP.

burs-stu/dhc