Early voting for the upcoming by-elections, including polls for the new Seoul and Busan mayors, is set to kick off this week for a two-day run, election officials said Thursday.

The advance polling will begin at 6 a.m. on Friday and last till Saturday evening, ahead of the election day on April 7.

Some 8.4 million voters in Seoul are eligible to cast ballots for a new mayor in the by-elections that come about a year ahead of the 2022 presidential poll.

In Busan, South Korea's second-biggest city, slightly more than 2.9 million voters are qualified to vote for a new mayor. Two regional administrative posts and 17 other seats on regional legislative councils will also be up for grabs in the elections.

No prior registration is needed for voters to cast their ballots during the two-day early voting period. The ballot boxes from early voting will be opened and counted along with those collected on the election day, once all the voting processes come to an end.

Young voters have traditionally shown a higher rate of participation in early voting, with a high turnout in early voting seen as more advantageous to progressive election candidates as opposed to conservative rivals.

One of the most recent opinion polls, conducted by Realmeter from Tuesday-Wednesday, showed Oh Se-hoon, the Seoul mayoral candidate from the conservative main opposition People Power Party (PPP), leading his rival from the ruling Democratic Party (DP), Park Young-sun, 57.5 percent to 36 percent.

Oh was leading Park in all age brackets, including voters in their 20s and 30s.

Previous opinion polls have also shown that well over 90 percent of eligible voters in Seoul are planning to cast their ballots in the Seoul mayoral election, seen as a potential game changer for the upcoming presidential election in March 2022.

In Busan as well, PPP mayoral candidate Park Hyung-jun was maintaining his solid lead over DP rival Kim Young-choon in recent opinion polls.

Starting Thursday through the election day, the release of the results of new opinion polls concerning the by-elections are prohibited under the official election law to block any election interference. (Yonhap)