The South Korean president, Yoon Suk Yeol, has apologised for his short-lived attempt to impose martial law this week, promising to face any legal consequences hours before parliament is due to vote on his impeachment.

In a televised address to the nation, Yoon said he was “very sorry” for causing public anxiety and inconvenience, and promised not to attempt to impose martial law a second time. He said he would let his People Power party (PPP) decide his term and would not avoid legal and political responsibility for his actions.

Immediately after his speech, the PPP leader, Han Dong-hun, said that the president’s early resignation was unavoidable and that he was no longer in a position to fulfil his duty, according to the national news wire Yonhap.

It was not clear whether the motion submitted by opposition lawmakers would get the two-thirds majority required for Yoon to be impeached. But it appeared more likely after Han on Friday called for suspending his constitutional powers, describing him as unfit to hold the office and capable of taking more extreme action, including renewed attempts to impose martial law.

Impeaching Yoon would require support from 200 of the National Assembly’s 300 members. The opposition parties that jointly brought the impeachment motion have 192 seats combined.

That means they would need at least eight votes from Yoon’s PPP. On Wednesday, 18 of its members joined a vote that unanimously cancelled martial law 190-0, less than three hours after Yoon declared the measure on television, calling the opposition-controlled parliament a “den of criminals” bogging down state affairs. The vote took place as hundreds of heavily armed troops encircled the National Assembly in an attempt to disrupt the vote and possibly to detain key politicians.

The turmoil resulting from Yoon’s bizarre and poorly-thought-out stunt has paralysed South Korean politics and sparked alarm among key diplomatic partners, including neighbouring 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.

Opposition lawmakers claim that Yoon’s martial law declaration amounted to a self-coup and drafted the impeachment motion around rebellion charges.

The PPP decided to oppose impeachment at a lawmakers’ meeting, despite pleas by Han, who isn’t a lawmaker and has no vote.

After a party meeting on Friday, Han stressed the need to suspend Yoon’s presidential duties and power swiftly, saying he “could potentially put the Republic of Korea and its citizens in great danger”.

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

Hong Jang-won, the first deputy director of South Korea’s National Intelligence Service, later told lawmakers in a closed-door briefing that Yoon called after imposing martial law and ordered him to help the defence counterintelligence unit to detain key politicians.

The targeted politicians included Han, the opposition leader, Lee Jae-myung, and the National Assembly speaker Woo Won Shik, according to Kim Byung-kee, one of the lawmakers who attended the meeting.

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

The former defence minister Kim Yong Hyun, who has been accused of recommending Yoon to enforce martial law, has been placed under a travel ban and faces an investigation by prosecutors over rebellion charges.

The defence vice-minister, Kim Seon Ho, who became acting defence minister after Yoon accepted Kim’s resignation on Thursday, has testified to parliament that it was Kim who ordered troops to be deployed to the National Assembly after Yoon imposed martial law.

Reuters and Associated Press contributed to this report