South Korean media say candidates from the largest opposition party are projected to win mayoral by-elections in Seoul and Busan. The races are widely seen as an important precursor to next year's presidential election.
Voting stations in the country's two largest cities closed shortly after 8 p.m. local time on Wednesday.
Public broadcaster KBS based its projections on joint exit polls with other TV stations and a prior phone survey.
In Seoul, Oh Se-hoon from the largest opposition People Power Party is projected to win the vote, beating Park Young-sun from the governing Democratic Party.
The projections also show in Busan, opposition candidate Park Heong-joon is on course to defeat the governing party candidate.
The election commission said voter turnout as of 8 PM was 55.1 percent in Seoul and 51.3 percent in Busan.
Vote counting is expected to continue until early Thursday morning.