A joint exit poll by three major TV broadcasters suggested the ruling Democratic Party (DP) won 14 of the 17 metropolitan mayoral and gubernatorial posts contested in Wednesday's local elections.
The main opposition Liberty Korea Party (LKP) secured only two seats, according to the poll released just after the vote ended at 6 p.m.
In parliamentary by-elections, held simultaneously with local elections for the first time, the DP was forecast to secure 10 of the 12 seats up for grabs, while the LKP probably won one post, it showed.
Voting took place for 12 hours at 14,134 polling stations nationwide. Ballot boxes were sealed after voting closed and were taken to 254 counting offices.
Results on who won are expected to emerge late Wednesday, although the outcomes for highly contested constituencies could come out early Thursday morning, election officials said.
Up for grabs are 4,016 local administrative, legislative and educational posts, including 17 metropolitan mayoralties and provincial governorships.
The biggest battleground is Seoul, where 20 percent of the country's population of 52 million live.
The exit poll suggested incumbent Mayor Park Won-soon, affiliated with the ruling DP, won the election against Kim Moon-soo of the conservative LKP and Ahn Cheol-soo of the minor opposition Bareunmirae Party. Park is seeking his third term.
The poll also indicated the ruling party's candidates won in 13 other major cities and provinces -- including the cities of Incheon, Busan and Ulsan and the provinces of Gyeonggi and South Gyeonsang. The LKP was ahead in only two of its strongholds -- the city of Daegu and North Gyeongsang Province.
The quadrennial local elections are seen as a barometer for President Moon Jae-in's liberal administration, which took office in May last year.
The governing DP has basked in a high approval rate on the back of strong public support for Moon and his drive to bring peace to the Korean Peninsula.
The LKP is still reeling from the collapse of public trust in it following last year's ousting of former President Park Geun-hye due to a corruption scandal. The party hoped to pick up the votes of "shy" conservatives who had maintained a low profile.
The local elections were held just one day after a historic summit between U.S. President Donald Trump and North Korean leader Kim Jong-un in Singapore.
Geopolitical developments have had limited impact on elections, but the latest unprecedented event is largely viewed as a boon for the ruling DP amid the mood for rapprochement.