South Korea's ruling party chief demanded on Friday that President Yoon Suk Yeol stand down over his martial law attempt, warning he posed a great danger to the country.

Han Dong-hoon said in televised remarks that credible evidence suggests that Yoon ordered the arrest of key politicians on the night he declared martial law this week.

"It was confirmed last night (Thursday) through credible evidence that on the day martial law was declared, President Yoon Suk Yeol instructed the chief of the Counterintelligence Command, his junior from university, to arrest key politicians, [labelling] them as anti-state forces, and mobilised intelligence agencies to arrest them," he said.

The stunning comments from the head of Yoon's People Power Party (PPP), almost guarantees enough lawmakers will vote to impeach the president on Saturday.

Han's comments were also a U-turn from Thursday, when he said he would block the impeachment, and another party leader insisted all 108 members of the PPP would unite to support Yoon in Saturday's vote.

But Han said on Friday that Yoon's refusal to acknowledge he had done anything wrong in declaring martial law on Tuesday night had prompted his change of position.

"Considering the newly emerging facts, I believe that a swift suspension of President Yoon Suk Yeol's duties is necessary to safeguard the Republic of Korea and its people," Han said.

Han said Yoon had not taken any personnel actions against military officials who had "illegally intervened". "Furthermore, he does not acknowledge that this illegal martial law is wrong," he added.

"Therefore, if President Yoon continues to hold the office of the presidency, there is a significant risk that extreme actions similar to the current state of emergency could be repeated, which could put the Republic of Korea and its citizens in great danger."

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 and the opposition quickly filed an impeachment motion, saying Yoon "gravely violated the constitution and the law". A vote is set for Saturday evening.

The opposition holds a large majority in the 300-member legislature and requires only a handful of defections from the PPP to secure the two-thirds majority needed for impeachment.

If the impeachment 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 take place within 60 days. (AFP)