A joint exit poll by three major TV broadcasters suggested the ruling Democratic Party won 14 of the 17 metropolitan mayoral and gubernatorial posts contested in Wednesday's local elections.
The main opposition Liberty Korea Party probably 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 possibly won only one, it showed.
Tentative turnout for the local elections was tallied at 60.2 percent, as 25.8 million of 42.9 million eligible voters cast ballots, according to the National Election Commission. It would mark the first time that the voting rate in a local election surpassed 60 percent since 1995.
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.
Final results are expected to emerge late Wednesday or early Thursday, election officials said.
Up for grabs are 4,016 local administrative, legislative and educational posts, including 17 metropolitan mayoralties and provincial governorships.
The DP expressed joy over the outcome of the exit poll.
"The public gave big support (for the government) in walking on the path toward peace and prosperity," said Choo Mi-ae, chairwoman of the DP.
Stung by a potential defeat, Hong Joon-pyo, the chief of the LKP, hinted at the possibility of his resignation from the post.
"The buck stops here," Hong wrote on his Facebook.
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.
Smaller opposition parties, including the center-right Bareunmirae, failed to secure chief posts in the 17 wide-area local entities, the poll showed.
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. (Yonhap)