South Korea’s president imposed martial law. Now comes a push to impeach him.

South Korea’s governing party chief expressed support Dec. 6 for suspending the constitutional powers of President Yoon Suk Yeol for imposing martial law this week, in a bombshell reversal that makes Mr. Yoon’s impeachment more likely.

Opposition parties are pushing for a parliamentary vote on Mr. Yoon’s impeachment on Dec. 7, calling his short-lived martial law declaration an “unconstitutional, illegal rebellion or coup.” But they need support from some members of the president’s People Power Party to get the two-thirds majority required to pass the impeachment motion.

The turmoil resulting from Mr. Yoon’s nighttime martial law decree has frozen South Korean politics and caused worry among neighbors, including fellow democracy Japan, and Seoul’s top ally, the United States, as one of the strongest democracies in Asia faces a political crisis that could unseat its leader.

During a party meeting, PPP leader Han Dong-hun stressed the need to suspend Mr. Yoon’s presidential duties and power swiftly, saying he poses a “significant risk of extreme actions, like reattempting to impose martial law, which could potentially put the Republic of Korea and its citizens in great danger.”

Mr. Han said he had received intelligence that Mr. Yoon had ordered the country’s defense counterintelligence commander to arrest and detain unspecified key politicians based on accusations of “anti-state activities” during the brief period martial law was in force.

“It’s my judgment that an immediate suspension of President Yoon Suk Yeol’s official duties is necessary to protect the Republic of Korea and its people,” Mr. Han said.

Impeaching Mr. Yoon would require support from 200 of the National Assembly’s 300 members. The opposition parties who jointly brought the impeachment motion have 192 seats combined. PPP has 108 lawmakers.

If Mr. Yoon is impeached, he would be suspended until the Constitutional Court rules on whether to remove him from office or restore his presidential power. Prime Minister Han Duck-soo, the country’s No. 2 official, would take over presidential responsibilities.

The Defense Ministry said it suspended from duty the defense counter intelligence commander, Yeo In-hyung, who Mr. Han alleged had received orders from Mr. Yoon to detain the politicians. The ministry also suspended Lee Jin-woo, commander of the capital defense command, and Kwak Jong-geun, commander of the special warfare command, over their involvement in enforcing martial law.

In a closed-door briefing to lawmakers, Hong Jang-won, first deputy director of South Korea’s National Intelligence Service, said Mr. Yoon called after imposing martial law and ordered him to help the defense counterintelligence unit to detain key politicians. The targeted politicians included Mr. Han, opposition leader Lee Jae-myung and National Assembly speaker Woo Won Shik, said Kim Byung-kee, one of the lawmakers who attended the meeting. Mr. Kim said Mr. Hong told lawmakers he ignored Mr. Yoon’s orders.

The spy agency’s director, Cho Taeyong, questioned Mr. Hong’s account. Mr. Cho told reporters that such an order would have come to him, rather than Mr. Hong, and that he never received any orders from Mr. Yoon to detain politicians.

Mr. Han earlier said he would work to defeat the impeachment motion even though he criticized Mr. Yoon’s martial law declaration as “unconstitutional.” Mr. Han said there was a need to “prevent damage to citizens and supporters caused by unprepared chaos.”

Thousands of protesters have marched in the streets of Seoul since Dec. 4, calling for Mr. Yoon to resign and be investigated. Thousands of autoworkers and other members of the Korean Metal Workers’ Union, one of the country’s biggest umbrella labor groups, have started hourly strikes since Dec. 5 to protest Mr. Yoon. The union said its members will start on indefinite strikes beginning on Dec. 11 if Mr. Yoon was still in office then.

South Korean Vice Defense Minister Kim Seon Ho promised the ministry’s “active cooperation” with an investigation by prosecutors into the military’s role in Mr. Yoon’s martial law enforcement. He said military prosecutors will also be involved in the investigation. He denied media speculation that Mr. Yoon and his military confidantes might consider imposing martial law a second time.

“Even if there’s a demand to enforce martial law, the Ministry of Defense and the Joint Chiefs of Staff will absolutely not accept it,” Mr. Kim said.

Mr. Kim became the acting defense minister after Mr. Yoon’s office on Dec. 5 accepted the resignation of Defense Minister Kim Yong Hyun, who has also been banned from traveling while he is investigated over the imposition of martial law.

Opposition parties and Mr. Han allege that it was Kim Yong Hyun who recommended that Mr. Yoon take the step. During a parliamentary hearing on Dec. 5, Kim Seon Ho said Kim Yong Hyun also ordered troops to be deployed to the National Assembly after Mr. Yoon imposed martial law.

Mr. Han leads a minority faction within the ruling party, and 18 lawmakers in his faction voted with opposition lawmakers to overturn Mr. Yoon’s martial law decree. Martial law ultimately lasted about six hours, after the quick overrule by the National Assembly forced Mr. Yoon’s Cabinet to lift it before daybreak Dec. 4.

The main liberal opposition Democratic Party leader Lee Jae-myung said in a televised speech on Dec. 6 that it was crucial to suspend Mr. Yoon as “quickly as possible.”

Mr. Lee said Mr. Yoon’s martial law enforcement amounted to “rebellion and also a self-coup.” He said Mr. Yoon’s move caused serious damage to the country’s image and paralyzed foreign policy, pointing to criticism from the Biden administration and foreign leaders canceling their visits to South Korea.

Mr. Yoon has made no immediate response to Mr. Han’s comments. He hasn’t made public appearances since he made a televised announcement that his martial law decree was lifted.

Prosecutor General Shim Woo Jung told reporters the prosecution plans to investigate rebellion charges against Mr. Yoon following complaints filed by the opposition. While the president mostly has immunity from prosecution while in office, the protection does not extend to allegations of rebellion or treason. It wasn’t immediately clear how the prosecution plans to proceed with an investigation on Mr. Yoon.

The Democratic Party is also considering filing a complaint against PPP floor leader Choo Kyung-ho, whom they accuse of attempting to facilitate Mr. Yoon’s martial law enforcement.

Mr. Choo, a Mr. Yoon loyalist, had asked party lawmakers to convene at the party’s headquarters rather than the National Assembly after martial law began. That meant fewer lawmakers were present for parliament’s vote on lifting martial law.

This story was reported by The Associated Press.