Early voting in South Korea's presidential elections kicked off on Thursday, with both main candidates set to cast ballots in a poll triggered by ex-leader Yoon Suk-yeol's ill-fated effort to suspend civilian rule last year.

All major polls have placed liberal Lee Jae-myung as the clear frontrunner in the race, with a recent Gallup survey showing 49 percent of respondents viewed him as the best candidate.

Trailing behind him is conservative ex-labour minister Kim Moon-soo of the ruling People Power Party – Yoon's former party – at 35 percent.

While election day is set for June 3, those who want to vote early can do so on Thursday and Friday – part of an initiative introduce in 2013 to help those unable to cast ballots on polling day or planning to travel.

South Koreans have in recent years turned out in larger and larger numbers for early voting, with 37 percent casting their ballots ahead of polling day in the 2022 presidential election.

Voting began at 6am for 12 hours, taking place again on Friday.

According to the National Election Commission, as of 9am the early voting turnout rate was 3.55 percent, the highest record yet for that time in South Korean election history.

"Some say power comes from the barrel of a gun, but I believe a vote is more powerful than a bullet," Lee told a rally on Wednesday.

According to a Gallup poll, more than half of Lee's supporters said they planned to vote early, compared to just 16 percent of Kim's supporters.

Kim has said he will cast his vote in Incheon, west of Seoul, with his campaign framing it as "the beginning of a dramatic turnaround", a nod to US General Douglas MacArthur's landing there during the Korean War.

Kim's decision to vote early has surprised many on the right, where conspiracy theories about electoral fraud – particularly during early voting – are rife.

The 73-year-old however reassured his supporters that there is "nothing to worry about."

"If you hesitate to vote early and end up missing the main election, it would be a major loss," said Kim on Wednesday.

"Our party will mobilise all its resources to ensure strict monitoring and oversight of early voting," he said. (AFP)