The advance voting began at 3,512 polling stations nationwide and will run until Saturday from 6 a.m. to 6 p.m., according to the National Election Commission.
Up for grabs are 4,016 local administrative, legislative and educational posts, including 17 metropolitan mayors and provincial governors. The by-elections will be held to fill 12 vacant parliamentary seats.
As of 1:00 p.m., turnout was 4.49 percent, according to the election watchdog.
It was higher than the 2.51 percent recorded on the first day of early voting for the 2014 local elections at 1:00 p.m.
The advance voting system was introduced for the by-elections in April 2013.
A poll released by the NEC showed Thursday that 76.5 percent of those surveyed said this week that they will definitely vote in the upcoming elections, up from 70.9 percent recorded two weeks earlier.
It said that 30.3 percent of those who intend to vote said they will cast ballots during the early voting.
President Moon and his wife Kim Jung-sook cast their ballots earlier in the day, joined by a number of senior secretaries, including chief of staff Im Jong-seok.
Rival parties have been ramping up their campaigns to boost turnout for the early voting. Higher turnout in the advance voting usually leads to a higher voting rate on election day.
Recent polls show that the ruling Democratic Party (DP) is taking a sizable lead in both elections.
The DP has been basking in a high approval rating on the back of strong public support for President Moon and his drive for inter-Korean rapprochement.
The party is calling for more participation in the early voting partly on concerns that young voters may skip the polls on the assumption that the DP will easily win.
"The June local polls are important elections in that voters'
support will determine the success of the Moon Jae-in administration," the ruling party said in a statement.
The main opposition Liberty Korea Party is still reeling from low public support following last year's ousting of former President Park Geun-hye due to a corruption scandal. It is hoping that conservative voters will come out after keeping a low profile on political issues.
The LKP appealed for support so as to judge the Moon administration, claiming that Moon's policy of increasing the minimum wage is hampering job growth.
The biggest battleground for the local elections is the capital Seoul, where 20 percent of the country's population of 52 million resides. The Seoul mayoralty carries extra weight as it is often deemed a stepping stone to the presidency.
Incumbent Mayor Park Won-soon is far ahead of his rivals with a support rating of 49.3 percent, according to a poll commissioned by three broadcasters. He was followed by Kim Moon-soo of the LKP with
13.6 percent and Ahn Cheol-soo of the minor opposition Bareunmirae Party with 10.7 percent.
Kim and Ahn failed to narrow their differences over a possible merger of their candidacies to compete against Park, the front-runner seeking a third term.
But it is still speculated that they may reach a last-minute agreement. Both sides are demanding that the other party drop their bid to make way for a single candidacy.
Ahn, a former presidential candidate, was a front-runner ahead of the 2011 by-election for Seoul mayor but dropped his bid and supported Park.
During TV debates aired late Thursday, Kim and Ahn lashed out at what they call Park's tepid performance in office for the past seven years.
Park earlier played down the impact of their potential merger of campaigns, saying the move would not appeal to voters.(Yonhap)