Sohn Hak-kyu, the leader of the main opposition Democratic Party (DP), scored a victory in the ruling party's stronghold of Bundang, just south of Seoul, which is considered the biggest match of Wednesday's by-elections, according to an exit poll by a local TV station.
Sohn's triumph, if confirmed, will boost his bid to become South Korea's next president, while dealing a blow to the Lee Myung-bak administration and the governing Grand National Party (GNP) ahead of next year's general elections and presidential polls.
The former governor of Gyeonggi Province garnered 54.2 percent, leading his contender Kang Jae-sup, a former GNP leader, who earned
44.5 percent, according to the poll released by the news channel YTN immediately after voting was closed at 8 p.m. The margin of error was plus or minus 1.7 percentage points.
A victory for the 64-year-old, who defected from the ruling party four years ago, would be an apparent boon for the presidential hopeful. Sohn had said he staked his political career on the ballot test.
The final turnout in Bundang is expected to be over 40 percent, more than the average of 32.8 percent recorded in by-elections held since 2000, the National Election Commission said.
Three parliamentary seats and one gubernatorial post are on offer in the elections, with the other 34 races vying for small local government heads and council members.
Official election results are expected to be available late Wednesday night.