In a lengthy televised address on Thursday, he alleged North Korea had hacked South Korea's election commission, casting doubt on his party's landslide election defeat in April.
Yoon, whose country has Asia's fourth-largest economy, hopes political allies will rally to support him but this appeared less likely after his address. The leader of his ruling People Power Party (PPP) said the time had come for Yoon to resign or be impeached by parliament.
Protesters are staging rallies to call for the president to be impeached for imposing martial law. (AP PHOTO)
Late on Wednesday, six opposition parties led by the Democratic Party submitted a bill for Yoon's impeachment to parliament. A vote is expected on Saturday, a week after the first one failed because most of the PPP members boycotted.
At least seven members of the party were expected to support the new impeachment motion. At least eight PPP votes are needed for the two-thirds majority required to impeach Yoon.
Yoon said the opposition was "dancing the sword dance of madness" by trying to drag a democratically elected president from power, nine days after his aborted attempt to grant sweeping powers to the military.
"I will fight to the end," he said. "Whether they impeach me or investigate me, I will face it all squarely."
The remarks were his first since he apologised on Saturday and said he would leave his fate in the hands of his party.
His defiant new comments raise the possibility that Yoon, a career prosecutor and a legal expert, may have decided to take his chances to court, hoping to make a comeback.
"It appears that he just doesn't want to step down and is trying to hang in there as he still thinks he did the right thing," said Shin Yul, a Myongji University political science professor.
A vote to impeach Yoon would send the case to the Constitutional Court, which has up to six months to decide whether to remove him from office or reinstate him.
In the latest sign that Yoon is losing his grip on power, PPP leader Han Dong-hoon told a meeting of party members on Thursday that they should join the opposition to impeach the president.
The party remains divided and Yoon still has the backing of some PPP lawmakers who oppose impeachment.
South Korean MPs have set up another vote to impeach the president after his martial law decree. (AP PHOTO)
Yoon is separately under criminal investigation for alleged insurrection over the December 3 martial law declaration, which he rescinded hours later, causing South Korea's biggest political crisis in decades.
In comments echoing his justification for declaring emergency rule in the first place, Yoon said "criminal groups" that have paralysed state affairs and disrupted the rule of law must be stopped at all costs from taking over government.
He was criticising the opposition Democratic Party which has blocked some of his proposals and demanded his wife be investigated over alleged wrongdoing. He gave no evidence of criminal activity.
A member of the Democratic Party leadership, Kim Min-seok, said Yoon's address was a "display of extreme delusion" and urged members of the president's party to join the impeachment vote.
with AP
