The election came after millions of South Koreans took to the streets in candlelit demonstrations to demand the removal of Park Geun-Hye, who was sacked by the country's top court in March over a corruption scandal and is now in custody awaiting trial.
Moon Jae-in, the presidential candidate of the Democratic Party of Korea, and his wife Kim Jung-sook (not pictured) greets supporters as they arrive at a polling station in Seoul, South Korea, May 9, 2017.
While Moon had 41.4%, his nearest challenger, the conservative Hong Joon-Pyo, got 23.3% of the votes.
If Moon's win is confirmed, he's expected to reshape Seoul's policy on North Korea, by challenging the deployment of the USA missile defense system, THAAD, and opening talks with Pyongyang.
Moon's expected victory in the race to the presidential Blue House in Seoul could herald an era of rapprochement with North Korea, and an unlikely meeting of minds with Donald Trump over Pyongyang's nuclear and ballistic missile programmes.
Moon's spokesman, Park Kwang-on, hoped for voter turnout of more than 80 per cent.
If Moon's victory is confirmed by vote tallies, he could change the country's position toward North Korea.
Voters said corruption and the economy were major issues going into the election, followed by North Korea.
South Koreans went to the polls Tuesday to choose a new president after Park Geun-Hye was ousted and indicted for corruption, and against a backdrop of high tensions with the nuclear-armed North.
The victor will be sworn in after the National Election Commission confirms the result Wednesday.
Tuesday's election saw strong turnout - about 77% of 42.5 million eligible voters. He didn't want to be named, citing office rules.
Multi-billion-dollar arms deal with Saudi Arabia
After leaving the Middle East, the president will meet Pope Francis at the Vatican. "Tolerance is the cornerstone of peace", Mr. Saudi Arabia's foreign minister Adel al-Jubeir also confirmed that an upcoming visit to the kingdom by U.S.
After voting, Hong said the election was a "war of regime choices between people, whether they decide to accept a North Korea-sympathizing leftist government or a government that can protect the liberty of the Republic of Korea", South Korea's formal name.
"I will be a president that can share a glass of soju with the public after work", he told reporters in April, referring to South Korea's vodka-like liquor.
The US and South Korea have a decades-long military and political alliance and Washington is by far Seoul's most important bilateral partner.
Hong, an outspoken former provincial governor who pitched himself as a "strongman", described the election as a war between ideologies and questioned Moon's patriotism.
This softer approach might put him at odds with South Korea's biggest ally, the United States.
The new South Korean president will still serve out a full, single five-year term.
He called for reforms to clean up social inequalities, excessive presidential power and corrupt ties between politicians and business leaders.
If Moon does become the next president, then, with the Democratic Party already holding 119 of the National Assembly's 300 seats, and with 46 more belonging to the centrist People's Party and Justice Party, which stands to the left of the Democrats, Moon has an enviable power base with which to steer the nation.
"I feel that not only my party and myself but also the people have been more desperate for a change of government", he said while casting his vote earlier on Tuesday.
Voters were mainly focused on corruption and the economy, but North Korea loomed large over the election after weeks of rising tensions between Pyongyang and the U.S. administration under President Donald Trump.
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