Embattled South Korean President Yoon Suk Yeol has defended his shock decision to declare martial law last week, saying he did it to protect the country's democracy.

In a surprise televised address on Thursday, he said the attempt was a legal decision to "prevent the collapse" of democracy and counter the opposition's "parliamentary dictatorship".

Yoon has suggested that he would not step down ahead of a second vote to impeach him in parliament on Saturday.

"I will stand firm whether I'm impeached or investigated," he said. "I will fight to the end".

The president and his allies are facing investigations on insurrection charges, and he and several of them have been banned from leaving South Korea.

On Thursday, the opposition-led parliament voted to impeach police chief Cho Ji-ho and justice minister Park Sung-jae. The two officials have been immediately suspended.

Unlike impeachment motions against presidents, which require 200 votes in the 300-strong National Assembly to be passed, other officials can be impeached with 150 votes.

In his address, his first since his apology over the weekend, Yoon denied that his martial law order was an act of insurrection, claiming that his political rivals were creating "false incitement" to bring him down.

Yoon repeated many of the same arguments that he used on the night he declared martial law: that the opposition was dangerous, and that by seizing control, he had been trying to protect the public and defend democracy.

However, Yoon added that he would not avoid his "legal and political responsibilities".

Last Saturday, an attempt by opposition lawmakers to impeach the president failed after members of his own ruling People Power Party (PPP) boycotted the vote. But opposition members are set to hold another impeachment vote this weekend and have vowed to hold one every Saturday until Yoon is removed from office.

The floor leader of the PPP, Choo Kyung-ho, stepped down after the failed impeachment attempt, and on Thursday the party elected Kwon Seong-dong, a Yoon loyalist, as his replacement.

Kwon told reporters on Thursday that he would hold discussions with PPP lawmakers on whether they should continue opposing Yoon's impeachment.

Yoon's party had been hoping to convince the president to leave office early, rather than force him out.

Minutes before Yoon spoke, his party leader Han Dong-hoon appeared on television saying it had become clear that the president was not going to step aside. Han then urged members of the party to vote to remove him from office this Saturday.

The opposition tabled another impeachment motion on Thursday, setting the stage for a vote at 17:00 local time (09:00 GMT) Saturday.

If South Korea's parliament passes the impeachment bill, a trial would be held by the Constitutional Court. Two-thirds of that court would have to sustain the majority for Yoon to be removed from office permanently.

Yoon has been a lame duck president since the opposition won the April general election by a landslide - his government has not been able to pass the laws it wanted and has been reduced to vetoing bills proposed by the opposition.

Yoon also accused North Korea sympathisers of trying to undermine his government when he declared martial law on the night of 3 December.

The announcement plunged the country into political turmoil. Protesters faced off against security forces in front of the National Assembly building while lawmakers scrambled to vote down Yoon's order.

Yoon withdrew his martial law order hours after it was blocked by lawmakers.

Since then, the country has remained on edge. There have been huge protests and strikes calling for Yoon's impeachment, and the presidential office was raided on Wednesday as Yoon faces multiple investigations on charges of insurrection and treason.

Meanwhile, ex-defence minister Kim Yong-hyun, who resigned and took responsibility for announcing martial law, attempted suicide while in detention on Tuesday, officials said. He is in a stable condition.