The South Korean ruling Grand National Party managed Thursday to win only six of the 16 key mayoral and gubernational posts in one of the biggest local elections in history.

In the elections widely seen as a mid-term assessment of the Lee Myung-bak administration, the Grand Nationals suffered an embarrassing setback as it only secured six of the 16 key mayoral and gubernatorial posts while its rival, the liberal main opposition Democratic Party (DP), secured seven.

The election came at a time when tensions are mounting on the Korean peninsula over the sinking of a South Korean warship, which Seoul believes was the work of Pyongyang. Seoul's conflicts with Pyongyang have always been a main selling point for the conservative Grand Nationals who often bill themselves as hawks, though the opposition camp upbraided them for using the deadly sinking of the warship, which claimed 46 lives, for political gains.

The surprising victory by the opposition camp proves that anti- Pyongyang sentiment is no longer enough to woo the voters wary of escalating tensions at home that could hurt the country's fledgling economic recovery, observers here said.

Still, the GNP won key battles in Seoul and Gyeonggi province. Incumbent Seoul Mayor Oh Se-hoon, a lawyer-turned-politician on the GNP ticket, narrowly defeated challenger Han Myeong-sook, a Democrat heavyweight, in a tense race whose fate remained undecided until early morning hours of Thursday.

Kim Moon-su, incumbent governor of Gyeonggi province and a potential presidential hopeful, also secured his second term in a tough campaign against Rhyu Shi-min, one of the closest confidants of Lee's liberal predecessor Roh Moo-hyun with a huge following among young voters.