"I will be president for all South Koreans," he told cheering supporters on Gwanghwamun Square in Seoul -- where vast crowds gathered over several months to demand the removal of his predecessor Park Geun-Hye over a huge corruption scandal.
Exit polls projected Moon would take 41.4 percent of the vote, 18 percentage points ahead of his nearest challenger.
"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.
South Korea was plunged into political turmoil by the scandal surrounding Park -- who is now in custody awaiting trial - and Moon promised to be a "president of cohesion" who also took care of those who did not support him.
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