South Korean President Yoon Suk Yeol's future remained uncertain Sunday even after surviving an impeachment vote over his brief imposition of martial law.

Yoon's party killed off an impeachment motion late Saturday even as huge crowds braved freezing temperatures to demand his ouster four days after he sent troops and helicopters to parliament.

But the deeply unpopular president's survival may be short-lived, with Yoon's People Power Party (PPP) saying that it had "effectively obtained (Yoon's) promise to step down" in exchange for blocking his impeachment.

"Through the orderly early resignation of the president, we will minimize the confusion to South Korea and its people, stably resolve the political situation and recover liberal democracy," PPP leader Han Dong-hoon said Sunday.

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

The premier vowed to "strengthen the emergency economic response system to closely examine risk factors in the financial and foreign exchange markets and respond promptly".

But the situation might not be sustainable, experts say, with the opposition declaring it will try to impeach Yoon again and calling for his arrest on Sunday, while protests are expected continue until his removal. 

"We will have a politically dead president -- basically unable to govern any longer -- and hundreds of thousands coming to the streets every week until Yoon is removed," Vladimir Tikhonov, Korean Studies professor at the University of Oslo, told AFP.

"Starting next week, candlelight protests will attract even larger crowds, the younger generation will rise up, and the PPP, living only for today, will inevitably implode with internal strife," columnist Park Chul-hyun said.

Sorry

On Saturday before the vote, Yoon, 63, reappeared for the first time in three days and apologized for the "anxiety and inconvenience". He said he would leave it to his party to decide his fate.

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

Many wore elaborate outfits, carrying home-made flags and waving colorful 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 defense minister on Wednesday and who was slapped with a travel ban, reports said.

Kim's arrest came about six hours after he showed up for questioning at 1:30 AM, Yonhap reported.

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.

The head of South Korean special forces said he had been ordered to "drag out" lawmakers from parliament.

A purported arrest list included opposition leader Lee Jae-myung, National Assembly Speaker Woo Won-shik, and PPP leader Han.

"Arrest all suspects involved in the insurrection, including those at the presidential office and the Ministry of National Defense, and raid all related organizations," three opposition parties said in a joint statement Sunday.