Sohn Hak-kyu, the leader of the main opposition Democratic Party, 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.

If the forecast is correct, Sohn's triumph 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 ahead of next year's general elections and presidential polls.

(Yonhap News)