South Korea's ruling and main opposition party vowed reforms Thursday after disenchanted younger voters rejected both of them and swept an independent candidate to victory in Seoul's mayoral election.

Civic activist Park Won-Soon, a political outsider, triumphed over Na Kyung-Won of the conservative ruling Grand National Party (GNP) by seven percentage points in Wednesday's vote.

The result was a blow to the conservatives' hopes of retaining power in a general election next April and a presidential poll in December 2012.

But the main opposition Democratic Party (DP) also found little cause for comfort at the result in the city of 10.4 million, almost a quarter of the country's population.

Park was backed by the DP and other left-leaning parties but stood as an independent, sidelining the DP in the process.

Exit polls showed about 70 percent of those aged under 50 supported him.

"Angry 2040 have impeached political status quo," read the headline in Dong-A Ilbo newspaper, referring to those aged in their 20s to 40s.

"A big tsunami hit party politics," said JoongAng Ilbo's main headline.

Incumbent GNP President Lee Myung-Bak, who is constitutionally barred from standing again, said he takes the party's defeat to heart and would "keep deeply in mind the will the young generation showed in this election".

GNP leader Hong Joon-Pyo promised party reform. "We will try to win people's hearts by making policies for those in their 20s and 30s, and try to communicate with them," he said.

The DP "needs to do deep soul-searching", its floor leader Kim Jin-Pyo told a party meeting, according to Yonhap news agency.

Social networks played a major role in Park's victory.

Young voters including celebrities posted tens of thousands of messages of support for him on sites like Twitter, urging peers to vote and to post photos of themselves at polling stations.

"Citizens won against power and their voting defeated the old era. Common sense and principle prevailed," Park, 55, said on his Twitter account.

Analysts said economic woes, a widening income gap, high college tuition fees and rising youth unemployment triggered the revolt by frustrated young voters.

"Young voters, although they make up a large part of the population, feel that there is no party to represent their interests and suffering," Yoon Pyung-Joong, Hanshin University professor of political philosophy, told Dong-A.

Park's victory despite his lack of political experience highlights distrust and anger at existing parties, Yoon Jong-Bin, a professor at Myongji University, told JoongAng.

The crushing GNP defeat is also "a clear voter judgement call" on Lee's administration, he said.

The mayoral vote was also a proxy battle between two top potential contenders in next year's presidential election.

Ahn Cheol-Soo, a software mogul who became an overnight political sensation when he expressed interest in running for mayor, later bowed out and gave his backing to Park.

Supporting Na was Park Geun-Hye, long seen as the GNP front-runner in 2012 and once dubbed the "queen of elections" for her campaigning skills. She now faces an uphill battle to shore up support, said Dong-A in an editorial.

"If the GNP can't win the hearts of those aged in the 20s and 40s, that will mean they are completely doomed at elections next year," it said.

The ruling party secured some consolation by winning eight out of 11 lower-level by-elections Wednesday while the DP garnered only two.