SEOUL (Reuters) - Liberal politician Moon Jae-in won South Korea's presidential election decisively on Tuesday, television networks declared, an expected victory ending nearly a decade of conservative rule and bringing a more conciliatory approach toward North Korea.
Moon's victory will end months of political turmoil that led to parliament's impeachment of conservative former President Park Geun-hye over an extensive corruption scandal. Park was the first democratically elected leader in South Korea to be removed from office, triggering a snap election to choose her successor.
The election was closely watched abroad at a time of high tension with North Korea, which is believed to be preparing for a sixth nuclear test and is working to develop a nuclear-tipped missile capable reaching the United States, presenting U.S. President Donald Trump with perhaps his most pressing security issue.
Moon's advocacy of engagement with North Korea contrasts with the approach adopted by the United States, South Korea's main ally, which is seeking to step up pressure on Pyongyang through further isolation and sanctions.
The White House nevertheless quickly congratulated Moon, saying it looked forward to working with him to strengthen the longstanding U.S.-South Korea alliance.
Climbing a temporary stage set up in the main square in downtown Seoul, Moon, a 64-year-old former human rights lawyer, who entered politics just five years ago, vowed in a midnight victory speech to usher in a new era for a country badly bruised by the scandal.
"I will make a just, united country," he said. "I will be a president who also serves all the people who did not support me."
With 80 percent of the votes counted at 1705 GMT, Moon was ahead with 40 percent, according to the National Election Commission. A conservative challenger, former prosecutor Hong Joon-pyo, was next with 25.5 percent followed by centrist candidate Ahn Cheol-soo with 21.4 percent.
A plurality of votes is enough for victory, but with his Democratic Party holding 40 percent of the single-chamber, 299-seat assembly, Moon will have to build coalitions to pass legislation.
The results were in line with an exit poll by South Korea's three biggest broadcasters, which showed Moon, 64, capturing 41.4 percent of the votes in a field of 13 candidates.
Voter turnout was 77.2 percent, the highest in 20 years, but short of the expected 80 percent mark amid drizzly weather.
A U.S. official said Moon's election could add "volatility" to ties with Washington, given his questioning of deployment of a U.S. missile defence there and his advocacy of engagement with North Korea.
But the official said Moon may moderate his stance on installation of the U.S. Terminal High Altitude Area Defense (THAAD) anti-missile system once he is in office and is not expected to significantly alter the alliance.
White House spokesman Sean Spicer congratulated Moon and said in a statement: "We look forward to working with President-elect Moon to continue to strengthen the alliance between the United States and the Republic of Korea."
Moon is expected to be sworn in for a five-year term later on Wednesday. He has said he would skip a lavish inauguration ceremony and start work straight away.
He is likely to quickly name a prime minister, who will need parliamentary approval. The main cabinet posts, including national security and finance ministers, do not need parliamentary confirmation.
Moon, who narrowly lost to Park in the last presidential election in 2012, has criticised the two previous conservative governments for failing to stop North Korea's weapons development. He advocates a two-track policy of seeking dialogue while maintaining pressure and sanctions to encourage change.
He also wants to reform powerful family-run conglomerates, such as Samsung and Hyundai, and boost fiscal spending to create jobs.
Daniel Russel, Washington's former top diplomat for Asia, told Reuters the differences between and Moon and Trump would mean inevitable friction, but "don't portend a crisis or failure" for relations.
"The important thing is the degree to which he (Moon) will consult, communicate and collaborate with the United States. Will he be a good alliance partner?" said Russel, now diplomat in residence at the Asia Society Policy Institute.
"Do you dial up the pressure until North Korea feels compelled to come to the table? Or do you dial down the pressure and turn on the mood music and light some candles until North Korea gets in the mood to talk. That's going to have to be sorted out."
North Korea's ambassador to Britain, Choe Il, set a defiant tone in an interview with Sky News, saying his country would continue with its nuclear and missile programs and conduct its sixth nuclear test "at the place and time as decided by our supreme leader, Kim Jong Un."
Moon was a close friend and confidant of late president Roh Moo-hyun, who served from 2003 to 2008 and advocated a "Sunshine Policy" of engaging North Korea through aid and exchanges.
Moon's campaign promises include a "National Interest First" policy that struck a chord with people who want South Korea to stand up to powerful allies and neighbours.
In a book he wrote published in January he said Seoul should learn to say "no to America" and on Tuesday said it should take a more active diplomatic role to curb North Korea's nuclear threat and not watch idly as the United States and China discussed the issue.
His victory was bolstered by strong support from younger people, according to the exit polls. Many of his supporters participated in rallies to demand that Park step down.
However, only 22 to 25 percent of people in their 60s and 70s voted for Moon, exit polls showed, underscoring a long-standing generation gap and wariness among older people about a less confrontational stance on North Korea.
Koh Yu-hwan, a professor at Dongguk University serving as a foreign policy adviser Moon, said conditions were not right for a reprise of the Sunshine Policy, given that Washington and Beijing were more hostile towards North Korea.
"Still, Moon is expected to engage in discussions, which could improve North-South relations," he said.
Moon's election could complicate the THAAD issue, which the former government in Seoul and the U.S. military deployed in spite of angry objections from China.
Moon has said the decision was made too quickly and the next administration should have the final say.
Russel said any move to reverse the deployment would not be easy and added that Moon had said he would not act unilaterally.
"It would be a matter of unwinding and removing a defensive system that is protecting both the United States and the Republic of Korea. That’s no small matter."
For graphic link on South Korea's presidential election, click: tmsnrt.rs/2p0AyLf
For graphic link on election demographics, click: tmsnrt.rs/2pGD25v
Additional reporting by Se Young Lee in Seoul and Matt Spetalnick and David Brunnstrom in Washington; Writing by Soyoung Kim and Jack Kim; Editing by Sandra Maler and James Dalgleish