South Korean President Yoon Suk Yeol on Saturday apoligised for his attempt to impose martial law this week and bowed during a televised speech to the nation, only hours head of a planned impeachment vote.

Yoon said he would not seek to avoid legal and political responsibility for his decision, which he said was born of desperation.

The speech was the embattled leader's first public appearance since he rescinded the martial law order early on Wednesday, just six hours after it was declared and after parliament defied military and police cordons to vote against the decree.

"I am very sorry and would like to sincerely apologise to the people who were shocked," Yoon said.

South Korea's ruling party leader Han Dong-hoon said after Yoon's address that the president was no longer in a position to carry out public duty and his resignation was now unavoidable.

On Friday Han said Yoon was a danger to the country and needed to be removed from power, increasing the pressure on Yoon to quit even though his People Power Party (PPP) members later reaffirmed a formal opposition to his impeachment.

Lawmakers will vote on the main opposition Democratic Party's motion to impeach Yoon later on Saturday.

Yoon shocked the nation late on Tuesday when he gave the military sweeping emergency powers in order to root out what he called "anti-state forces" and overcome obstructionist political opponents.

Some PPP members urged Yoon to resign before the vote, saying they did not want a repeat of the 2016 impeachment of then-President Park Geun-hye, who left office following months of candlelit protests over an influence-peddling scandal. Her downfall triggered the implosion of the party and a victory by liberals in presidential and general elections.

In scenes reminiscent of those protests, thousands of demonstrators holding candles assembled outside parliament on Friday night demanding Yoon's impeachment. More demonstrations are expected on Saturday ahead of the vote.

Prosecutors, the police and the Corruption Investigation Office for High-ranking Officials have all launched probes into Yoon and senior officials involved in the martial law decree, seeking to pursue charges of insurrection and abuse of power, among others.