Yoon Suk Yeol's televised statement on Thursday came hours before the main liberal opposition Democratic Party submits a new impeachment motion against Yoon.
The opposition party plans to put the motion on a floor vote on Saturday.
Its earlier attempt to impeach Yoon fell through last Saturday, with ruling party lawmakers boycotting a vote at the National Assembly.
Yoon's December 3 martial law decree has generated political chaos and protests calling for his ouster.
"I will fight to the end, to prevent the forces and criminal groups that have been responsible for paralysing the country's government and disrupting the nation's constitutional order from threatening the future of the Republic of Korea," he said.
Yoon said his martial law introduction was meant to defend South Korea's liberal democracy and constitutional order in the face of the liberal opposition party, which he says threatens the constitution.
Yoo said his martial law decree was an act of governance that cannot be the subject of investigations and doesn't amount to rebellion.
"The opposition is now doing a knife dance of chaos, claiming that the declaration of martial law constitutes to an act of rebellion. But was it really?" he said.
On Wednesday, Yoon's office resisted a police attempt to search the compound.
The main focus of the investigation is finding whether Yoon and other top military and government officials involved in imposing martial law committed rebellion. A conviction for rebellion carries a maximum penalty of death.
Earlier this week, Yoon's former defence minister was arrested on allegations of playing a key role in a rebellion and committing abuse of power. He became the first person formally arrested over the martial law decree.
with AP
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