Members of South Korea's ruling Democratic Party watch screens showing the result of exit polls of the Seoul mayoral by-election at the party headquarters in Seoul on April 7, 2021. (JUNG YEON-JE / POOL / AFP)

South Korean President Moon Jae-in’s ruling party suffered a devastating defeat in a special election for key mayoral posts amid political scandals and policy blunders, vote counts showed on Thursday.

Millions of South Koreans went to the polls on Wednesday to elect chiefs of the country’s two largest cities, the capital Seoul and port city of Busan, among 21 local offices up for grabs.

Millions of South Koreans went to the polls on Wednesday to elect chiefs of the country’s two largest cities, the capital Seoul and port city of Busan, among 21 local offices up for grabs

The election was widely seen a key barometer for potential political shifts for Moon’s progressive party with less than one year left before the March 9 presidential election.

Moon and his Democratic Party have seen their approval ratings plunge to record lows in recent months amid skyrocketing housing prices, deepening inequality and sex abuse scandals.

READ MORE: South Koreans vote for key city offices as political winds shift

In Seoul, People Power contender Oh Se-hoon secured 57.5 percent of votes among 8.4 million voters, clinching victory over Democratic candidate Park Young-sun who garnered 39.2 percent, according to the state election commission.

Exit polls had predicted Oh’s landslide victory. Vote counts showed that Oh - who previously served as Seoul mayor from 2006 to 2011 - won all 25 districts of the city, fetching three times as many as Park got in the affluent town of Gangnam.

Oh expressed gratitude to voters after his victory was finalised, while Park conceded defeat, vowing “soul-searching over punishment from citizens.”

People Power Chairman Kim Chong-in said after exit polls were released that popular anger had “exploded” over the government’s policy failures.

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“I cannot hold back the sense of responsibility that’s already weighing heavily on my mind,” Oh said.

In Busan, People Party candidate Park Hyung-joon received 62.7 percent of the votes, beating Democrat Kim Young-choon who earned 34.4 percent.

Voter turnout was 58.2 percent in Seoul and 52.7 percent in Busan from some 12.16 million eligible to cast ballots, exceeding 50 percent in a snap election for local offices for the first time, according to the commission.