He cast his vote in Seoul on Tuesday and told reporters later, "I gave all my body and soul (to the election) to the very end".
Park is in jail, on trial, but denies any wrongdoing.
These days Moon Jae-in often greets more like a South Korean pop idol than a 64-year-old former human rights lawyer.
Some analysts say North Korea has toned down its provocations toward the South ahead of the election to avoid emboldening the country's hard-liners.
(AP Photo/Ahn Young-joon). South Korean presidential candidate Hong Joon-pyo of the Liberty Korea Party shakes hands with his supporters during an election campaign in Seoul, South Korea, Monday, May 8, 2017.
The North has carried out two nuclear tests and a series of missile launches since the start of past year in its quest to develop a missile capable of delivering a nuclear warhead to the U.S. mainland.
Mr Moon has advocated greater dialogue with the North while maintaining pressure and sanctions, in contrast to Ms Park who cut nearly all ties.
If Moon's win is confirmed, he is expected to overhaul Seoul's policy on North Korea, looking to open talks with Pyongyang and challenge the deployment of a controversial USA missile defense system.
Moon had a relatively low share of the total vote - 41.4 percent according to an exit poll - but there were many more major candidates than in 2012, when Park won 51.5 percent, beating Moon by about a million votes.
The new president of South Korea will take office on May 10, immediately after the confirmation of the results by the NEC. Most candidates, including Moon and Ahn, plan to skip a lavish inauguration ceremony if successful and start their new job right away.
"Moon has been criticized for running his own "clique", so in order to get over that negative image, he'll seek cooperation to avoid rifts and conflicts", Kim said.
Polls close at 20:00 local time (11:00 GMT), with the victor expected to be announced soon after.
Said 24-year-old voter Chung Yu Jin: "This election wouldn't have taken place if there weren't people who took to the streets every weekend last winter".
A decisive win by Moon will provide much-needed stability and is expected to improve market sentiment at a time when robust exports have supported a recovery in Asia's fourth-largest economy. He has now positioned himself as an alternative to Mr Moon, promising to be tougher on national security.
The election is being closely watched overseas at a time of high tension with North Korea, which is believed to be preparing for a sixth nuclear test and has vowed to test an intercontinental missile. Pyongyang carried out a fifth test in September and is believed ready for another.
As policy in South Korea looks like shifting, many sceptics remain who think that no amount of talking to Kim Jong-un will persuade him to renounce either his despotic power or his nuclear ambitions. He has stopped short of overtly criticising Trump's aggressive tone during the most recent crisis on the Korean peninsula, and has declared that he and the United States president are "on the same page" in regarding the Obama administration's policy of "strategic patience" as a failure.
Japan's Chief Cabinet Secretary Yoshihide Suga told a regular news conference the relationship between South Korea and Japan would develop regardless of who became president.
Despite the global focus on North Korea, the biggest priorities for South Koreans are the economy and corruption.
A Moon Jae-in presidency would represent a real shift in attitude towards North Korea. Its official Rodong Sinmun newspaper said on Tuesday conservative rule in the South should be "buried in the grave of history".
"To do so, the puppet conservative group's move to remain in power should be definitely crushed".
Tillerson, Ivanka Trump and Jared Kushner are lead Trump administration officials in favor of sticking with the Paris agreement. Former president Barack Obama, meanwhile, had endorsed Mr Macron in a video message uploaded online.
Our guys have kind of bought into that, so I'm excited to see how we play and whoever I choose to play in those minutes. Washington coach Scott Brooks said he respects the league's decision and figured a suspension could be coming.
Canadian Imperial Bank of Commerce (NYSE:CM), from Financial sector has been on the upside of the performance spectrum. Canadian Imperial Bank of Commerce had its "mkt perform" rating reiterated by analysts at Keefe, Bruyette & Woods.
In other Zendesk news, Director Caryn Marooney sold 1,693 shares of the business's stock in a transaction on Thursday, March 30th. Perhaps, that suggests something about why 2.40% of the outstanding share supply is held by institutional investors.
It's also a further dent to the populist tide that resulted in Britain's exit from European Union and Donald Trump's victory. European Commission President Jean-Claude Juncker congratulated Macron on Twitter .
Mr Macron, who hopes to revitalise the Franco-German axis at the heart of the European Union , plans to visit Berlin shortly. Macron spoke with Merkel after his victory was announced, telling her that he would travel to Berlin "very quickly".
The U.S. force would also be bolstered by requests for matching troops from North Atlantic Treaty Organisation nations. At the same time, the strategy would retain the previous administration's reconciliation policy with the Taliban.
The prosecution told the court that the investigation revealed that the victim was kidnapped and later gang-raped in a vehicle . Rajesh Chaudhary, the fourth accused, turned an approver as he was involved in rape, but did not have any role in murder.
The rapper, whose real name is Curtis Jackson, no longer lives at the mansion and has been trying to sell it for over a year. FARMINGTON, Conn. (AP) - Authorities say they have arrested a suspected burglar at rapper 50 Cent's mansion in CT .
Celtic won the game against Rangers 5-1 . "For me it was great to win the appeal because it was never a red card", said Brown. When the switch failed to stem Celtic's attacking tide he sent on Joe Garner for Emerson Hyndman, a forward for a midfielder.
Stanton is up to 20 homers at Citi Field, the most of any visiting player at the ballpark, and has six multi-homer games there. For most of last season, the 31-year-old Cabrera battled knee trouble, but still finished second on the team with 141 games.
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The exit polls were conducted for 13 hours to 7 p.m. local time (1000 GMT) at 330 polling stations with around 89,000 voters. A strong turnout is expected when South Koreans make their way to the voting stations today. (Han Jong-chan/Yonhap via AP).
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But Environmental Protection Agency Administrator Scott Pruitt has said called the Paris pact "a bad deal for America". Secretary of State Rex Tillerson said at his Senate confirmation hearing in January he supports staying in the deal.