A former opposition leader often mentioned as a potential presidential candidate announced his retirement from politics Thursday after suffering a humiliating defeat in parliamentary by-elections a day earlier.

Sohn Hak-kyu, a former four-term lawmaker and ex-leader of what is now the main opposition New Politics Alliance for Democracy (NPAD), lost to Kim Yong-nam, a political novice of the ruling Saenuri Party, in Wednesday's by-elections seen as a midterm referendum on President Park Geun-hye's conservative government.

The ruling party won a landslide victory, taking 11 of the 15 seats up for grabs.

Sohn had run in District D of Suwon, just south of Seoul, which has never elected an opposition candidate since the current electoral system was introduced in 1988.

"I am leaving politics today," Sohn said in a hastily arranged news conference at the National Assembly. "It is my long-held belief that a politician should speak through elections. I was not chosen by voters in the July 30 by-elections. I humbly accept that will."

Sohn, who currently serves as an adviser to the party, has often been mentioned as a potential presidential candidate of the opposition camp.

In 2012, he competed in his party's primaries for the December presidential vote but eventually lost to Moon Jae-in, a former chief of staff to late former President Roh Moo-hyun.

Wednesday's defeat, however, dealt a blow to the politician amid uncertainties about his influence within the opposition camp.

In South Korea, a number of big-name politicians have announced their retirement from politics after being defeated in elections, although not all have kept their word.

Late former President Kim Dae-jung announced his retirement upon losing the presidential election in 1992, only to return to win the next election in 1997. (Yonhap)