SEOUL (REUTERS, AFP, THE KOREA HERALD/ASIA NEWS NETWORK) - Former human rights lawyer Moon Jae In is poised to take office as South Korea President on Wednesday (May 10) after securing what exit polls showed to be a landslide victory in Tuesday's election, ending nearly a decade of conservative rule and bringing a more conciliatory approach towards North Korea.

His closest rivals, conservative Hong Joon Pyo and centrist Ahn Cheol Soo, conceded defeat late on Tuesday even before the official vote count is out.

The concessions followed exit polls that forecast Moon would win by a landslide, ending a decade of conservative rule in South Korea and setting up a sharp departure from recent policy toward nuclear-armed North Korea.

Moon told his celebrating Democratic Party members and supporters that he would push for reform and national unity if he is confirmed as the next president

"I will build a new nation. I will make a great Korea, A proud Korea. And I will be the proud president of such a proud nation," he said at a victory rally in Seoul's Gwanghwamun area - where vast crowds gathered over several months to demand the removal of his predecessor Park Geun Hye over a huge corruption scandal.

Exit polls projected Moon would take 41.4 percent of the vote, 18 percentage points ahead of his nearest challenger.

"This is a great victory of great people who stayed with me to create a country of justice... where rules and common sense prevail," he said.

South Korea was plunged into political turmoil by the scandal surrounding Park - who is now in custody awaiting trial - and Moon promised to be a "president of cohesion" who also took care of those who did not support him.

He has pledged a softer touch with North Korea and tougher action against family-run conglomerates that dominate Asia's fourth-biggest economy.

Moon's stance on North Korea could put him at odds with United States President Donald Trump, who has stressed that he could take military action to halt the isolated nation's nuclear ambitions.

The son of North Korean refugees, Moon criticised the early installation of a US missile shield on South Korean soil and has said he would meet Kim Jong Un under the right circumstances.

At home, Moon faces the task of healing a nation which is still reeling from the graft probe that culminated in Park's arrest in March after months of street protests.

He is expected to add fiscal stimulus to create jobs for disaffected youth and bolster an economy forecast to expand this year at the slowest pace since 2012.

After casting his vote on Tuesday, Moon issued a call for unity.

"I myself will be the first to make such efforts by embracing other candidates and their parties for harmony," he said. "I hope the people do what they can do until the end of the election but come back as one after the election for the nation's unity."

Exit polls conducted by the three terrestrial broadcasters - KBS, MBC and SBS - indicated that the liberal Democratic Party of South Korea candidate received 41.4 per cent of the votes. Polls closed at 8 pm and ballot counting will continue to Wednesday morning.

If the figures are correct, it will mark the return of a liberal administration after just under 10 years of conservative governments.

The official outcome of the election, based on the vote-count, is expected to be announced early Wednesday morning.

Despite the rain, voters cast their ballots at 13,964 polling stations nationwide.

More than half the population had voted as of 2pm, according to the National Election Commission.

The nation's new president is expected to be revealed around 2am to 3 am on Wednesday (May 10) South Korean time.

Ballot selfies were a trend this year, with even celebrities taking pictures of their hands marked with the voting stamp.

"I took a photo of us holding hands with the stamps showing because I wanted to remember the moment. We made this election possible," said Kong Young Hee, a 29-year-old office worker. "It was more meaningful because it was the first time I cast my ballot with my wife after we got married."

Twitter, Facebook and Instagram were flooded with "voting selfies" from early in the morning, as voters marked the occasion and celebrate their participation in democracy.

This is the first presidential election since the National Election Commission lifted a ban on such photographs, as part of its efforts to boost turnout.