South Korea’s ruling party says it will block a move by the opposition to impeach the beleaguered president, Yoon Suk Yeol, as police said they were investigating Yoon for alleged insurrection over his botched attempt to impose martial law.

The floor leader of the People Power party vowed on Thursday that its lawmakers would “unite” to defeat the opposition-led motion to impeach the deeply unpopular leader.

“All 108 lawmakers of the People Power party will stay united to reject the president’s impeachment,” Choo Kyung-ho told a livestreamed party meeting.

Opposition lawmakers need eight ruling party lawmakers to vote with them for the impeachment bill to pass. The opposition says the vote is expected on Saturday.

Yoon now faces multiple investigations, after South Korea’s prosecutor general on Thursday ordered prosecutors to directly investigate the president and other key officials for alleged insurrection over the martial law declaration. Police are conducting a separate parallel investigation.

Both agencies will investigate Yoon, former Defence Minister Kim Yong-hyun, and martial law commander Park An-su, among others. Prosecutors have already imposed a travel ban on Kim, who resigned as defence minister over his involvement in Tuesday’s martial law order. South Korean presidents do not have immunity from the crime of insurrection, which can carry the death penalty.

Separately, prosecutors have imposed a travel ban on Kim Yong-hyun, who resigned as defence minister over his involvement in Tuesday’s martial law order. Opposition lawmakers have accused Kim of attempting to flee the country, a claim that Kim reportedly dismissed as “political agitation”.

Both the prosecutors’ office and the high-ranking corruption investigation office are reviewing whether they have authority to directly investigate the insurrection allegations or must transfer the cases to police.

It remained unclear on Thursday, though, whether enough members of Yoon’s party would give him the backing needed to avoid impeachment proceedings, as the embattled president sought to deflect criticism of his actions.Yoon plunged the country into political turmoil on Tuesday evening after he declared martial law in a surprise, late-night televised address, saying it was needed to safeguard the country’s from “anti-state forces” and “threats posed by North Korea”. He did not provide details of the threats.

Within hours he was forced to rescind the order when parliamentarians defied an attempted military blockade and assembled to vote it down.

On Thursday, his health minister, Cho Kyoo-hong, said he disagreed with Yoon’s declaration, although he had taken part in a cabinet meeting shortly before the president’s shock announcement.

Cho told a parliamentary session the declaration had been illegal and unconstitutional, but claimed he could not recall if ministers had opposed Yoon at the cabinet meeting. “I was so surprised and flustered,” he said, according to the Yonhap news agency. “To be frank, I do not remember who said what.”

On Thursday morning, Yoon accepted the resignation of his defence minister Kim Yong-hyun and nominated his ambassador to Saudi Arabia, Choi Byung-hyuk, as the new defence minister.

As Yoon has attempted to contain the fallout, more details have emerged of the confusion leading up to one of the most dramatic episodes in the modern history of South Korea, which is Asia’s fourth-largest economy and a key US ally.

Following the declaration, which attempted to ban political activity and censor the media in South Korea, armed troops attempted to force their way into the National Assembly building in Seoul, only to stand back when parliamentary aides sprayed them with fire extinguishers.

Details emerged that high-profile critic and former radio host Kim Ou-joon was forced into hiding after troops were deployed to his YouTube studio in central Seoul.

Around 10 soldiers blocked access to his broadcasting studio shortly after martial law was declared, while another military unit reportedly visited his home with an arrest warrant, prompting his escape to an undisclosed location.

The vice-defence minister, Kim Seon-ho, claimed he had not been told in advance about plans, adding the decision to send troops to the national assembly building had come from the now departed defence minister.

Kim Seon-ho said he had opposed the troop deployment and disagreed with Yoon’s description, made during his televised address, of the assembly as a “den of criminals”.

The US deputy secretary of state, Kurt Campbell, said Yoon had “badly misjudged” the martial law decision, which took the White House by surprise.

He said South Korea would be “in a challenging place” in the next few months and the US goal would be to make clear its alliance with Seoul is “absolutely rock solid.”

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Opposition parties need a two-thirds majority to pass the impeachment bill. If it passes, South Korea’s constitutional court will then decide whether to uphold the motion – a process that could take up to 180 days fraught with potential complications due to the court currently having only six justices instead of its full nine-member bench.

“The people and the aides who protected parliament protected us with their bodies. The people won, and it’s now time for us to protect the people,” said Democratic Party lawmaker Kim Seung-won.

“We need to immediately suspend the authority of President Yoon. He has committed an indelible, historic crime against the people, whose anxiety needs to be soothed so that they can return to their daily lives.”

If Yoon were to be suspended from exercising power, prime minister Han Duck-soo would fill in as leader.

If the embattled president resigned or was removed from office, a new election would be held within 60 days.

There was no immediate reaction from North Korea to the drama in the South.

Yoon had been embraced by leaders in the west as a partner in the US-led effort to unify democracies against growing authoritarianism in China, Russia and elsewhere.

But he caused unease among South Koreans by branding his critics as “communist totalitarian and anti-state forces”. In November, he denied wrongdoing in response to influence-peddling allegations against him and his wife, and he has taken a hard line against labour unions.

Reuters contributed to this report