South Korean media say candidates from the largest opposition party have taken big leads in mayoral by-elections in Seoul. In Busan, the candidate from the largest opposition party is projected to win.
These 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 expected to secure 59 percent of the vote, topping Park Young-sun from the governing Democratic Party.
In Busan, opposition candidate Park Heong-joon is projected to win.
The election commission said voter turnout as of 8 p.m. was 55.1 percent in Seoul and 51.3 percent in Busan.
Vote counting is expected to continue until early Thursday morning.