Yoon shocked the nation and his own ruling People Power Party on Tuesday when he announced he was imposing martial law in order to root out "anti-state forces" and overcome obstructionist political opponents.
He reversed course about six hours later after parliament, including some members of his party, voted to oppose the decree.
Speaking after a party meeting at the parliament on Friday, party leader Han Dong-hoon said Yoon had ordered the arrest of prominent politicians over grounds that they were among those "anti-state forces" during the martial law.
The main opposition Democratic Party has scheduled an impeachment vote for Saturday evening, and the national police have launched an investigation into Yoon on claims of insurrection filed by an opposition party and activists.
On Thursday, the ruling party said it was against impeachment, but Han suggested that stance may be shifting in light of "credible evidence" that Yoon had intended to arrest and detain political leaders at Gwacheon, just south of Seoul.
"I said yesterday that I would try not to pass this impeachment in order to prevent damage to the people and supporters caused by the unprepared chaos, but I believe that President Yoon Suk Yeol's immediate suspension of office is necessary to protect the Republic of Korea and its people in light of the newly revealed facts," Han said.
He did not explicitly call for impeachment or respond to reporters when asked for clarification.
Democratic Party leader Lee Jae-myung said the declaration of martial law was a rebellion waged by the president in order to maintain or extend his power.
"It's an act of insurrection," he said. "It's a pro-military coup."
