The Republic of Korea's President Yoon Suk-yeol promised "reform "and the head of his ruling party resigned on Thursday, after the main opposition won a landslide victory in a parliamentary election.

Prime Minister Han Duck-soo and all senior presidential secretaries, except those in the Office of National Security, offered to resign to take responsibility for the election defeat, the presidential office said.

"I will strive to reform state affairs and stabilize the economy and improve people's livelihoods," said Presidential Chief of Staff Lee Kwan-seop, delivering Yoon's message in a televised briefing.

Yoon said he would humbly accept the will of the people in the general election, Lee said.

With all votes counted, the liberal opposition Democratic Party, or DP, won 161 out of 254 directly contested constituency seats, while Yoon's ruling People Power Party, or PPP, obtained 90 seats, tentative results from the National Election Commission showed.

Including proportional seats, the DP and its proportional representation satellite party won 175 seats in the 300-member National Assembly.

Also in the media spotlight is former justice minister Cho Kuk, whose newly launched liberal party won 12 seats.

The PPP, together with its satellite party, won a total of 108 seats.

Despite its overwhelming victory, the opposition fell short of securing a supermajority of 200 seats that would have enabled it to overturn presidential vetoes, rewrite the constitution and even impeach the president.

It still means that Yoon, in the remaining three years of his single five-year term, will struggle to pursue his agenda with the country's executive and legislative branches split between two political parties in a divided government.

DP leader Lee Jae-myung said he is grateful for the people's support and called winning the general election not only a victory for the party, but also "a great victory for the people".

"The DP will actively take the lead in solving problems related to people's livelihoods," he said at a disbanding ceremony for the party's election committee on Thursday.

The bitterly fought race was seen by some analysts as a referendum on Yoon, who has suffered low ratings for months amid a cost of living crisis and a spate of political scandals.

"The election results have starkly reflected the public's strong dissatisfaction with the first two years of President Yoon's administration, placing him in a very difficult predicament," Yoo Sung-jin, a professor at Scranton College of Ewha Womans University, said.

The election result showed that voters primarily hold the president and the ruling party accountable for the current political and economic situation, Yoo said.

"Without a significant change in governance style, implementing the president's agenda will be challenging," he said.

Pushing forward agenda

Despite heightened polarization between the two main parties and a National Assembly dominated by the opposition, Lam Peng Er, principal research fellow of the Korea Centre at the National University of Singapore's East Asian Institute, said Yoon may still push forward his policy agenda.

"I will not be so pessimistic by saying that the Yoon administration is doomed to become a lame duck for the next three years," Lam said. "It really depends on the skill of the leaders of the People Power Party to negotiate with the other parties."

Noting that general elections mainly focus on domestic issues, Lam said he expects Yoon to remain powerful in setting the direction for foreign policy.

Yoon will likely continue his policy to reinforce relations with the United States and Japan, while keeping a hard-line stance toward Pyongyang, Lam said, adding that the opposition-led legislature will still have a say in the national budget for diplomatic activities and defense.

Agencies contributed to this story.