South Korean President Lee Myung-bak has suffered a setback in local polls held amid tensions over the sinking of a navy ship in March.
With almost all the votes counted, official results show the opposition Democratic Party (DP) won seven mayoral and governor posts.
Mr Lee's Grand National Party (GNP) won only six posts.
The result is seen as a blow for Mr Lee's tough stance on North Korea, accused of sinking the Cheonan.
Forty-six sailors died when the warship went down on 26 March.
An international investigation said it was torpedoed by North Korea, but Pyongyang denies involvement and had urged voters in the South to punish Mr Lee's government for his stance.
North and South Korea have cut all ties since the sinking, amid rising tensions on the peninsula.
South Koreans were voting for some 4,000 mayors, governors and local government representatives in the elections.
The poll was widely seen as a mid-term referendum on Mr Lee's leadership and a key gauge for public sentiment ahead of the 2012 presidential contest.
Commentators had predicted that the tension with North Korea would favour Mr Lee's conservative government.
But the GNP won only six out of 16 mayoral and gubernatorial races across the country - half of its total from four years ago.
Turn-out was at 54.5%, the highest for local polls in 15 years. Large numbers of young people were said to have voted.
The BBC's John Sudworth in Seoul says that perhaps this group, thought to be more liberal in its outlook, chose to register its protest against a conservative president and his tough anti-North Korean stance.
The chairman of the ruling GNP, Chung Mong-joon, and Secretary-General Chung Byung-kook have both offered to resign over the party's defeat.
"We humbly accept the voice of the Korean people of rebuke," Chung Mong-joon told a meeting. "I want to take this chance to express my wish to resign."
The opposition DP party has claimed victory in the elections, calling it a "public verdict against the Lee government's arrogance and self-righteousness".