South Korean lawmakers vote Saturday on whether to impeach President Yoon Suk Yeol over his failed martial law bid, in a second parliamentary showdown that remains too close to call.

Thousands are expected to rally in Seoul outside the National Assembly, which will vote at around 4:00 PM (0700 GMT) on an impeachment resolution for "insurrectionary acts" — a week after a first attempt to remove Yoon for the martial law debacle failed.

Yoon has vowed to fight "until the very last minute" and doubled down on unsubstantiated claims the opposition is in league with the country's communist foes.

Two hundred votes are needed for the impeachment to pass, meaning opposition lawmakers must convince eight parliamentarians from Yoon's conservative People Power Party (PPP) to switch sides.

Seven ruling party lawmakers have pledged to support it.

Local media is reporting that many are still making up their minds.

The main opposition Democratic Party on Saturday said that a vote for impeachment was the "only way" to "safeguard the Constitution, the rule of law, democracy and South Korea's future."

"We can no longer endure Yoon's madness," spokeswoman Hwang Jung-a said.

Rallies supporting impeachment are scheduled to begin near the parliament around midday.

Organisers have promised to distribute food and banners to the protesters to lift their spirits in the freezing December temperatures.

And K-pop singer Yuri of the band Girl's Generation — whose song Into the New World has become a protest anthem — said she had pre-paid for food for fans attending the rally.

"Stay safe and take care of your health!" she said on a superfan chat platform.

And one protester said she had rented a bus so that parents at the rally could use it to change diapers and feed their babies.

An act of insurrection

Should his impeachment be approved, Yoon would be suspended from office while South Korea's Constitutional Court deliberates.

Prime Minister Han Duck-soo would step in as the interim president.

The court would then have 180 days to rule on Yoon's future. 

If it backs his removal, Yoon would become the second president in South Korean history to be successfully impeached.

But there is also precedent for the court to block impeachment. In 2004, then-president Roh Moo-hyun was removed by parliament for alleged election law violations and incompetence, but the Constitutional Court later reinstated him.

The court currently only has six judges, meaning their decision must be unanimous.

And should the vote fail, Yoon can still face "legal responsibility" for the martial law bid, Kim Hyun-jung, a researcher at the Korea University Institute of Law, told AFP.

"This is clearly an act of insurrection," she said.

"Even if the impeachment motion does not pass, the president's legal responsibilities under the Criminal Code... cannot be avoided."

So angry

Yoon has remained unapologetic and defiant as the fallout from his disastrous martial law declaration has deepened and an investigation into his inner circle has widened.

On Friday, prosecutors said they had arrested a military commander who headed the Capital Defence Command. 

The Seoul Central District Court also issued arrest warrants for the national police chief and the head of the city's police, citing the "risk of destruction of evidence".

Yoon's approval rating — never very high — has plummeted to 11 percent, according to a Gallup Korea poll released Friday.

The same poll showed that 75 percent now support his impeachment.

Protesters calling for his ouster run the gamut of South Korean society — from K-pop fans waving glowsticks to retirees and workers.

"Impeachment is a must and we must fight relentlessly," Kim Sung-tae, a 52-year-old worker at a car parts manufacturer, told AFP.

"We're fighting for the restoration of democracy."