South Korea's liberal party candidate, Lee Jae-myung, was elected president in Tuesday's snap election, six months to the day after he evaded military cordons to vote against a shock martial law decree imposed by his ousted predecessor.

Lee's victory stands to usher in a political sea change in Asia's fourth-largest economy, after the backlash against the martial law brought down Yoon Suk Yeol, the conservative outsider who narrowly beat Lee in the 2022 election.

Nearly 80 percent of South Korea's 44.39 million eligible voters cast their ballots, the highest turnout for a presidential election in the country since 1997, with Lee terming the polls "judgment day" against Yoon's martial law and the People Power Party's failure to distance itself from that decision.

With more than 99 percent of the votes counted, the Democratic Party's Lee stood at 49.3 percent to PPP candidate Kim Moon-soo's 41.3 percent, according to National Election Commission data.

A subdued Kim conceded the race and congratulated Lee in brief remarks to reporters.

Lee had long been favoured to win, and his supporters erupted in cheers as exit polls by the country's major broadcasters showed him defeating Kim by wide margins. (Reuters)