The candidate from the main opposition Democratic Party won a crucial local by-election in Seoul, the voting results showed Thursday, capping a high-stakes contest seen as a key barometer of voter sentiment ahead of next year's parliamentary election in April.

Jin Kyo-hoon of the Democratic Party had 56.52 percent of the vote to represent Gangseo Ward, while his rival from the ruling People Power Party, Kim Tae-woo, had 39.37 percent, according to the National Election Commission. Jin was 17.15 percentage points ahead of Kim.

Voter turnout was tallied provisionally at 48.7 percent, the NEC said.

Jin called his election a "victory for common sense and principle" and pledged to use his "second and minute" for the district.

Even before the final results came, Kim conceded defeat in the by-election.

"I feel sorry that I could not respond to the backing of those who supported me," he said in a statement. "I also extend my appreciation to people in my camp who did their utmost despite the difficult circumstances."

A graduate of the Korean National Police University, Jin served in various top police posts, including as the chief of the Jeonbuk Provincial Police.

Although the election will only decide who will become one of Seoul's 25 ward chiefs, it has gained much attention, with the turnout for the two-day advance voting recording the highest ever in the country's by-election history.

During the early voting on Friday and Saturday, 113,313 out of the 500,603 eligible voters cast their ballots, representing a turnout of 22.64 percent, the NEC said.

A total of six candidates were running in the by-election, including Kim, a former ward chief, and Jin, a former deputy chief of the national police.

The by-election took place after Kim was removed from the top post of the ward office in May due to a suspended prison sentence for leaking secrets he gained while working for a special inspection team under former President Moon Jae-in.

Although the seat was vacated due to Kim's conviction, the People Power Party named him as its candidate after a special presidential pardon in August, which reinstated his right to run in elections.

Rival parties have each made a final plea for voters' support for their respective candidates, with the People Power Party saying Kim is the right person who will be working for the Gangseo constituency.

The Democratic Party, meanwhile, has appealed for support for Jin, arguing that winning the seat will serve as an opportunity to showcase judgment of the Yoon Suk Yeol administration. (Yonhap)