At stake are 4,016 local administrative, legislative and educational posts, including 17 metropolitan mayoralties and provincial governorships. The by-elections are to fill 12 vacant parliamentary seats.
Voting kicked off at 6:00 a.m. and was to run until 6:00 p.m. at 14,134 polling centers nationwide, according to the National Election Commission. Eligible voters number 42.9 million.
As of 3 p.m., the voter turnout rate reached 50.1 percent, up 4.1 percentage points from four years earlier, according to data from the election watchdog.
The reading includes the 20.14 percent of voters who cast their ballots in two-day early voting last week.
The higher-than-expected turnout for the advance voting earlier drew keen attention to whether the overall rate for the local elections could breach the 60 percent mark for the first time since 2010.
The turnout rate for the 2014 local elections came to 56.8 percent, while the turnout for early voting in those elections was 11.5 percent.
Wednesday's voting marked the first time that parliamentary by-elections and local elections were held at the same time.
The ruling Democratic Party is widely expected to clinch a sweeping victory on the back of strong public support for Moon and his drive for peace with North Korea.
Rival parties have been gauging the impact of the historic summit between US President Donald Trump and North Korean leader Kim Jong-un, which was held just a day earlier in Singapore.
The unprecedented event appeared to have already been factored into the local elections, but rapprochement on the Korean Peninsula is viewed as a boon for the liberal DP.
The LKP is hoping that "shy" conservative voters will come out after keeping a low profile on political issues.
The opposition party is still reeling from low public support following last year's ousting of former President Park Geun-hye due to a corruption scandal. (Yonhap)