Moon Jae-in gestures to his party members as they watch a broadcast of exit polls in Seoul.
The Democratic Party's Moon Jae-in is expected to win the South Korean presidential election, according to an exit poll conducted by the country's three major broadcasters.
With 9 percent of votes counted as of 10:30 p.m. local time (1330 GMT), Moon garnered 37.5 percent support.
Nilsson-Wright said Moon appealed to a scandal-weary electorate as a clean politician who could reunite the country following the drawn-out Park scandal, which has seen the former president accused of taking or demanding $52 million in bribes from big businesses.
Daniel Pinkston, an worldwide relations expert at Seoul's Troy University, said Moon's policy toward the North reflected "a strong public desire for a reduction in tensions and animosity between the two Koreas".
Because the vote is a by-election to replace the jailed former president Park, the new president will be sworn in as soon as the vote count is completed and a victor formally announced.
Former prosecutor Hong Joon-pyo was second with 23.3 percent of the votes and centrist candidate Ahn Cheol-soo had 21.1 percent. A plurality is enough for victory.
Voting stations are set to close at 8 p.m. and South Korean TV stations plan to release the results of their joint exit polls soon after the vote ends.
On the square, freelancer Koh Eun-Byul, 28, told AFP: "I am so happy because now there is hope for some meaningful change".
Hong said in a televised address that he will accept the election result and be satisfied with restoring his party.
Voter turnout was 77.2 per cent - the highest in 20 years - but fell short of the predicted 80 per cent mark, most likely due to the drizzly weather.
"It would be advisable that the next South Korean leader respond positively to China's proposal to resume dialogues - Pyongyang suspends its nuclear programme in exchange for the U.S".
He will be sworn in for a five-year term on Wednesday and has pledged to get straight down to work, skipping the usual lavish inauguration ceremony. The main cabinet posts, including national security and finance ministers, do not need parliamentary confirmation.
Moon was chief of staff for the last liberal President Roh Moo-hyun, who sought closer ties with North Korea by setting up large-scale aid shipments to the North and by working on now-stalled joint economic projects.
Many analysts say Moon, if elected, likely won't pursue drastic rapprochement policies because North Korea's nuclear program has progressed significantly since he was in the Roh government a decade ago.
His Democratic Party holds 40 percent of the single-chamber, 299-seat assembly, which means he will have to build coalitions to pass legislation.
Only 22-25 percent of people in their 60s and 70s voted for Moon, exit polls showed, underscoring a long-standing generation gap. The North has been wary of having another conservative administration in the South.
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.
North Korean leader Kim Jong-un said in his News Year's message that his country has entered the final stage of preparing to launch an intercontinental ballistic missile (ICBM) capable of hitting the United States.
Moon, whose campaign promises include a "National Interest First" policy, has struck a chord with people who want the country to stand up to powerful allies and neighbours.
But a resumption of the factory zone, once hailed as the symbol of inter-Korean reconciliation, is a tricky issue due to the possibility of South Korea violating United Nations sanctions resolutions against the North, analysts say.
A big challenge for whoever wins will be U.S. President Donald Trump, who has proven himself unconventional in his approach to North Korea, swinging between intense pressure and threats and offers to talk.
Moon believes better inter-Korean relations is the best way to provide security. "It would take months for these tensions to sort themselves out, especially if Trump publicly criticizes the new South Korean president or penalizes Seoul, perhaps through trade measures", wrote Scott Seaman, director of Asia at think tank Eurasia Group, in a note to clients.
But for many South Korean voters, corruption, slowing growth, unemployment, and even air pollution from China top the list of concerns.
One issue on which they do differ remains the controversial Terminal High Altitude Area Defense (THAAD) missile defense system, deployed by the USA on South Korean soil and switched on last week.