The two leaders of the main opposition party quit Thursday over a crushing defeat in parliamentary by-elections seen as a midterm referendum on President Park Geun-hye's conservative administration.

The move by Kim Han-gil and Ahn Cheol-soo came a day after New Politics Alliance for Democracy (NPAD) gained only four out of the 15 seats at stake.

"I humbly accept the people's will. We lost the elections that we should have won. I am sorry for it," Kim said in a news conference at the parliament.

Ahn, who had jointly shared the party's top post with Kim for only four months, also said he assumes responsibility for the election results and vowed to do his best for the party, even though he becomes a rank-and-file lawmaker.

The election rout left the main opposition party in disarray over its future and dealt a blow to Ahn's apparent bid to run for president in 2017.

Ahn, the founder of South Korea's largest anti-virus software firm, AhnLab, had a strong following especially among young Koreans, due largely to his clean and upright image when he entered the 2012 presidential campaign, of which he later dropped out.

Still, Ahn's political capital has been squandered as he failed to demonstrate his leadership he gained through a merger in March with the then main opposition Democratic Party.

Meanwhile, a resounding victory by Park's ruling Saenuri Party raised the number of its seats to 158 in the 300-member National Assembly. The unexpected development could give a much-needed boost to Park, who has vowed to push through her reform agenda following April's deadly ferry disaster.

Holding the majority means that the ruling party can push bills through the parliament unilaterally without cooperation with the main opposition party.

Park's office said it will further step up its efforts to help recover the economy and innovate the country, noting that the election victory of the ruling party is people's call for the top two items on the agenda.

Park has pledged to remodel South Korea from square one to help make it a safer country after the government came under intense fire for its bungled response to the sinking that left more than 300 people dead or missing, mostly high school students. (Yonhap)