Hundreds of thousands of South Koreans cast their ballots Thursday in advance voting for the presidential election set to be held next week, the election watchdog said.

More than 590,000 voters have already voted as of 9 a.m., according to the National Election Commission (NEC), with many more expected to follow before the two-day voting period ends at 6 p.m. Friday. There is a 12-hour overnight break starting from 6 p.m. Thursday.

Among early voters were National Assembly Speaker Chung Sye-kyun and key officials from all major political parties here.

Dozens of citizens, including armed service members, stand in lines to cast their ballots at a polling station set up in Seoul's Yongsang train station on May 4, 2017, the first day of a two-day early voting period for the presidential election slated for next week. (Yonhap)

Dozens of citizens, including armed service members, stand in lines to cast their ballots at a polling station set up in Seoul's Yongsang train station on May 4, 2017, the first day of a two-day early voting period for the presidential election slated for next week. (Yonhap)

It marked the first early voting for a presidential election as the system was introduced in 2014 as a way of boosting voter turnout.

Unlike absentee voting that only allows those likely to be absent on election day to vote in advance, early voting allows any eligible voter to do so regardless of reason.

In the 2014 local elections, 11.5 percent of all voters or 20.2 percent of those who actually cast their ballots did so during the early voting period. The ratio increased to 21 percent of all votes cast in the National Assembly elections held last year, according to the NEC.

Over 42.4 million people are eligible to cast ballots in the upcoming election, marking the largest number of voters in the country's history, the NEC noted earlier.

To help further boost voter turnout, the election commission said it has also set up polling booths at major ports, including the Incheon International Airport and Seoul's major train stations, in anticipation of people going away on trips over the three-day weekend created by Children's Day that falls on Friday.

Absentee voting for people with disabilities and those away from the country aboard ships has been held since Monday and was set to end later Thursday. A six-day voting period for voters staying overseas was held late last month.

The five leading presidential candidates pose for a picture before the start of the sixth and last round of TV debates held May 2, 2017. They are (from L) front-runner Moon Jae-in of the Democratic Party, Hong Joon-pyo of the Liberty Korea Party, Yoo Seong-min of the Bareun Party, Sim Sang-jeung of the Justice Party and Ahn Cheol-soo of the People's Party. (Yonhap)

All votes cast in advance will be kept sealed with those locked overseas to be brought here for official counting following the end of the presidential election on Tuesday, the NEC has said.

The election watchdog is also seeking to boost the turnout rate of Internet-savvy voters, allowing online posts of photographs confirming their participation in the election for the first time.

Previously, posting such photos had been prohibited out of concern over possible election law violations. The NEC said pictures showing actual ballot papers will continue to be banned, along with pictures of other people at polling stations with possible publicity issues.

The captured image shows a photo posted by South Korean pop singer BoA on a social network service account to prove her participation in early voting for the upcoming presidential election in Seoul on May 4, 2017, the first day of a two-day voting period for the election slated for May 9, 2017. (Yonhap)

The captured image shows a photo posted by South Korean pop singer BoA on a social network service account to prove her participation in early voting for the upcoming presidential election in Seoul on May 4, 2017, the first day of a two-day voting period for the election slated for May 9, 2017. (Yonhap)

The upcoming election marks a rare presidential by-election caused by the March 10 ouster of former President Park Geun-hye over corruption allegations that have also led to her arrest and formal indictment.

With the top executive office currently vacant, the single five-year term of the next president will begin immediately following his or her election.

Currently, 13 hopefuls are vying for the presidential office after two candidates dropped out of the race.

The latest poll on the presidential election published Wednesday showed Moon Jae-in of the liberal Democratic Party in the lead with 42.4 percent, followed by two runner-ups -- Ahn Cheol-soo of the center-left People's Party and Hong Joon-pyo of the former ruling Liberty Korea Party -- who were tied at 18.6 percent.

The other major candidates included Sim Sang-jeung of the progressive Justice Party, who in the latest poll stood in fourth place with 7.3 percent, and Yoo Seong-min of the splinter conservative Bareun Party with 4.9 percent. (Yonhap)

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