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Early results suggested that Moon, the Democratic Party's candidate in the election to replace ousted president Park Geun-hye, won by a substantial margin, reports the CNN.
The election of Moon, a 64-year-old former human rights lawyer, is likely to mean an overhaul for Seoul's policy on North Korea as he favors engagement with Pyongyang and has challenged the deployment of US missile defense system, THAAD, on South Korean soil.
"This is the great victory for the great people who have been with me to build a just country, united country and a country where principle and common sense works," said Moon.
With around 34 per cent of the vote counted, Moon had 39.36 per cent of the vote followed by conservative Hong Jun-pyo at 26.71 percent and centrist Ahn Cheol-soo with 21.8 percent.
Park, who narrowly beat Moon in 2012 to become South Korea's first female president was impeached for sharing government documents with her confidante Choi Soon-sil who held no official government position.
Choi is accused of using her relationship with Park to accumulate millions of dollars in donations to her foundations.