A former pro-democracy activist and human rights lawyer, Moon Jae-In of the Democratic Party -- who favours engagement with Pyongyang -- has been leading opinion polls for months.
The final Gallup Korea survey of the campaign ahead of today's vote gave him 38 per cent, far ahead of centrist Ahn Cheol-Soo on 20 per cent.
Today's vote was called to choose a successor to Park Geun-Hye after her impeachment for corruption and abuse of power.
Moon has promised to reinvigorate the South's sluggish growth and create more jobs, and hinted at a more flexible approach towards its nuclear-armed neighbour.
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The front-runner advocates dialogue and reconciliation with the North to defuse the situation and eventually lure it into negotiations that have been at a standstill for years -- an approach criticised by his conservative opponents.
Pyongyang yesterday slammed the South's conservatives -- who have been in power for a decade -- as "senseless traitors seeking only confrontation and war" who were responsible for the "tragic" state of North-South relations.
The North has carried out two nuclear tests and a series of missile launches since the start of last year and Washington has said military action was an option, sending tensions spiralling.
Moon -- who lost to Park in the last election in 2012 -- has benefited electorally from the anger over the scandal that brought her down, which saw millions of South Koreans taking to the streets in candlelit demonstrations to demand her removal.
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