South Korean presidential candidate Moon Jae-in (C) of the Democratic Party reacts as he watches screens showing the result of exit polls of the presidential election at a hall of the National Assembly in Seoul on May 9, 2017.
Mr Moon had a relatively low share of the total vote - 41.4% according to an exit poll - but there were many more major candidates than in 2012, when Ms Park won 51.5%, beating Mr Moon by about a million votes.
National elections are public holidays in South Korea and preliminary figures showed a turnout of 77.2 percent - the highest for 20 years in a presidential poll.
Moon has criticized the two former conservative governments for failing to stop North Korea's weapons development.
Moon, the child of refugees who fled North Korea during the Korean War, will lead a nation shaken by the scandal that felled Park Geun-hye, whose criminal trial is scheduled to start later this month.
The phone call between Moon and Lee took place at 8:10 a.m. (2310 GMT), Lee's spokesman said.More news: EPA chief outlines legal-case recusals
South Koreans have long protested the USA army's presence in their country, regularly demonstrating outside of bases. "And I will be the proud president of such a proud nation", Moon was quoted by Yonhap as telling his supporters in Seoul earlier.
The eventual victor Moon Jae-in was shown first, acting out Daenerys Targaryen's dragon-riding scene in the arena, while Ahn Cheol-soo was portrayed as Jon Snow and Sim Sang-jung blared a horn to kick off a duel between the two front-runners of the race.
"This gives South Korea a ideal opportunity to take control over Korean Peninsula issues, particularly given the concerns that it's been excluded from discussions due to the leadership void", Kim said.
"He has to pursue cooperation with other liberal and centrist parties, since the Democratic Party does not have the majority", said Kim Man-heum, head of the Korea Academy of Politics and Leadership.
South Korea may see a sharp departure from recent policy under Moon, who favors closer ties with North Korea.
Moon's North Korean parents resettled in South Korea's southeast before he was born in January 1953.More news: Congress grilling United over Dao; CEO Munoz apologizing
"This is a great victory of great people who stayed with me to create a country of justice. where rules and common sense prevail", he said. "In particular, we must continue to take action to counter ongoing aggression by Pyongyang, including its missile and nuclear programs".
"South Koreans are more concerned that Trump, rather than North Korean leader Kim Jong Un, will make a rash military move, because of his outrageous tweets, threats of force and unpredictability", Duyeon Kim, a visiting fellow at the Korean Peninsula Future Forum in Seoul, wrote recently in Foreign Affairs magazine.
Park is now in jail awaiting trial on 18 charges, including bribery, over allegations that she took money from billionaire Samsung scion Lee Jae-yong in an effort by Lee to facilitate a major business transaction.
Mr Moon, Ahn and other candidates promised to reform the chaebols, which dominate the economy and have always been criticised for operating with little scrutiny.
Another issue is relations with top trading partner Beijing, which imposed a series of measures seen as economic retaliation over the deployment of a U.S. anti-missile system, THAAD, in the South.
A decisive win by Moon, with more than 40 percent of votes in a field of 13 candidates, would end months of political turmoil stemming from a parliamentary vote in December to impeach former President Park Geun-hye over an extensive corruption scandal.More news: Former Attorney General: Alarm about Russian blackmail led to warning on Flynn
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