The new president will face the challenge of rallying a society deeply scarred by the attempt at military rule and an export-heavy economy reeling from unpredictable protectionist moves by the United States, a major trading partner and a security ally.
After being impeached by parliament in December, Yoon was removed from office by the Constitutional Court on April 4, less than three years into his five-year term, triggering the snap election that stands to remake South Korea's political leadership and foreign policies.
As of mid-afternoon (local time), 30.5 million people, or nearly 69 per cent of the electorate, had voted at 14,295 locations, according to the National Election Commission.
Turnout was running slightly ahead of the 2022 presidential vote with polls set to remain open until 9pm AEST and following early voting when more than a third of the 44.39 million eligible voters cast their ballots.
"Only six golden hours are left to save South Korea, which is in crisis due to the greed of the establishment," liberal frontrunner Lee Jae-myung said as he urged people to vote in a Facebook post.
Both Lee and his conservative rival Kim Moon-soo have pledged change, saying a political system and economic model set up during South Korea's rise as a budding democracy and industrial power are no longer fit for purpose.
Their proposals for investment in innovation and technology often overlap, but Lee advocates more equity and help for mid-to low-income families while Kim has campaigned on giving businesses more freedom from regulations and labour strife.
Overshadowing any social policy initiatives, however, is Yoon's brief attempt to impose martial law that has loomed large over the poll.
Lee has called the election "judgment day" against Kim and his People Power Party, accusing them of having condoned the martial law attempt by not fighting harder to thwart it and even trying to save Yoon's presidency.
Kim was Yoon's labour minister when the former president declared martial law on December 3.
Kim has branded Lee a "dictator" and his Democratic Party a "monster," warning if the former human rights lawyer becomes president, nothing will stop them from working together to amend laws simply because they do not like them.
"I and the People Power Party will do our best to save people's livelihoods and the economy," Kim said in a Facebook post.
The frontrunner Lee and his rival Kim cast their ballots during early voting last week.
Yoon and his wife voted at a school near their private residence on Tuesday, appearing relaxed but ignoring questions as they left the polling station.
There were no female candidates running in Tuesday's election for the first time in 18 years.
Despite polls showing wide gaps between young men and women, gender equality was not among the key policy issues put forward during the election, a stark contrast from the 2022 vote.
The National Election Commission is scheduled to certify the result on Wednesday and the winner's inauguration is expected within hours. There will be no presidential transition as the office has remained vacant since Yoon was removed.
Reuters
