Liberal politician Moon Jae-in decisively won South Korea's presidential election on Tuesday, television networks declared, an expected victory ending nearly a decade of conservative rule and bringing a more conciliatory approach toward North Korea.

Mr 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, which a court upheld in March.

Ms Park became the first democratically elected leader in South Korea to be removed from office, triggering a snap election to choose her successor.

Climbing a temporary stage set up in the main square in downtown Seoul, a beaming Mr Moon surrounded by his Democratic Party leaders, vowed to usher in a new era for a country badly bruised by the scandal in a midnight victory speech.

"I will make a just, united country," he told a crowd gathered to see the former human rights lawyer who entered politics just five years ago.

"I will be a president who also serves all the people who did not support me." With 80 per cent of the votes counted at 5pm GMT, Mr Moon was ahead with 40 per cent, according to the National Election Commission.

A conservative challenger, former prosecutor Hong Joon-pyo, was next with 25.5 per cent followed by centrist candidate Ahn Cheol-soo with 21.4 per cent. The results were in line with an exit poll by South Korea's three biggest broadcasters, which showed Mr Moon, 64, capturing 41.4 per cent of the votes in a field of 13 candidates.