The new president has always been an advocate of rapprochement between the two Koreas and has advocated reopening two joint projects that provided North Korea with valuable streams of hard currency - a step that would run directly counter to Mr. Trump's strategy of tightening sanctions.
But Moon boasts a clean image himself, said Kim Neung-Gou, president of online newspaper Polinews, and has been "riding on waves of protests against Park and accumulated corruption".
Polls in South Korea closed at 7 a.m. ET, although the final results won't be known until later. Unlike President Donald Trump, who has maintained a hard line on the reclusive nation, Moon is in favor of dialogue with it.
South Korean presidential candidate Moon Jae-in, center left, of the Democratic Party speaks as his wife Kim Jung-suk stands next after voting for a presidential election at a junior high school in Seoul, South Korea, Tu. The system has angered Beijing, which says its powerful radars allow Washington to spy on its own military operations. We are the owners of the Korean peninsula problem; we have to lead, right?
"If necessary, I will fly straight to Washington", he said. "I will go to Beijing and Tokyo and under the right circumstances go to Pyongyang as well".
Moon, whose victory capped one of the most turbulent political stretches in the nation's recent history and set up its first liberal rule in a decade, assumed presidential duties after the National Election Commission officially declared him as victor.
Already unhappy with a slowing economy and shrinking opportunities for the young, South Koreans took to the streets by the hundreds of thousands previous year when President Park Geun-hye was accused of conspiring with a friend to extort bribes from the country's big conglomerates.
"Moon's Democratic Party (DPK) not only lacks a majority in parliament but his policy proposals are nowhere near bold enough to address the challenges facing the economy", they argued in a note.
But a year after Roh's unexpected election victory in 2002, Moon joined the administration as a presidential aide, tasked with weeding out official corruption and screening candidates for top government posts, before rising to become his chief of staff.
"In face of the impeachment and arrest of an incumbent president, our people opened the path toward the future for the Republic of Korea", said Moon, referring to South Korea's formal name.
Taking up duties as commander in chief, Moon received a call from Army Gen.
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Basuki Tjahaja Purnama, the governor of Indonesia's capital city, Jakarta , was given a two-year prison sentence on Tuesday. The two-year prison sentence came as somewhat of a surprise as prosecutors had recommended a two-year suspended sentence.
Prime Minister Shinzo Abe said he looked forward to meeting the new leader, adding: "Japan and South Korea are facing common challenges in East Asia, led by responses to the North Korean issue". He then left with his wife, Kim Jeong-sook, for a national cemetery in Seoul.
Moon says Roh's death led him to politics. South Korea's relations with Japan are strained by the Japanese military's sexual exploitation of South Korean women during World War II, and relations with China have been irritated over the deployment of the THAAD missile-defense system.
Moon's election could add volatility to relations with Washington, given his questioning of the deployment of the US military's Terminal High Altitude Area Defense (THAAD) anti-missile system, but is not expected to significantly change the alliance, a USA official said.
"I will quickly move to solve the crisis in national security".
The election body had finished counting votes as of 6 a.m. and said Moon gathered 41 per cent of the votes, comfortably edging conservative Hong Joon-pyo and centrist Ahn Cheol-soo, who gathered 24 per cent and 21 per cent of the votes, respectively.
Hong, the conservative, is an outspoken former provincial governor who pitched himself as a "strongman", described the election as a war between ideologies and questioned Moon's patriotism.
Park's trial on bribery, extortion and other corruption charges could send her to jail for life if she is convicted. Dozens of high-profile figures, including Park's longtime confidante, Choi Soon-sil, and Samsung's de facto leader, Lee Jae-yong, have been indicted along with Park.
When the corruption scandal involving Park flared last fall, Moon saw his popularity rise amid massive public outrage toward her conservative government.
Moon, a liberal former human rights lawyer who was jailed as a student by a previous dictatorship, favors closer ties with North Korea, saying hard-line conservative governments did nothing to prevent the North's development of nuclear-armed missiles and only reduced South Korea's voice in global efforts to counter North Korea.
This softer approach might put him at odds with South Korea's biggest ally, the United States.