Seoul, South Korea: The projected winner of South Korea's presidential election is a former special forces soldier, pro-democracy activist and human rights lawyer.

An exit poll forecast a landslide victory for left-leaning Moon Jae-In of the Democratic Party, giving him 41.4 percent support, 18 percentage points ahead of his nearest challenger.

Victory will cap a political career that began with student activism in the days of military rule, when he was convicted of taking part in illegal protests.

The election came after millions of South Koreans took to the streets in candlelit demonstrations to demand the removal of Park Geun-Hye, who was sacked by the country's top court in March over a corruption scandal and is now in custody awaiting trial.

The irony is that he was once chief of staff to liberal president Roh Moo-Hyun, who committed suicide in 2009 after being questioned over graft allegations.

"Corruption is the biggest issue in South Korean politics," says Robert Kelly of Pusan National University. "That's absolutely true. Every South Korean president has gotten into trouble for corruption and bribery and graft and things like that, of varying degrees."

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".

- Arrested and expelled -

Moon was born on the southern island of Geoje in 1952 during the Korean War after his parents fled the North.

His father was a menial worker at a prisoner-of-war camp while his mother peddled eggs in the nearby port city of Busan, with the baby Moon strapped to her back, the politician wrote in his autobiography.

He entered law school in Seoul in 1972 but was arrested and expelled for leading a student protest against the authoritarian rule of dictator Park Chung-Hee -- the ousted president's father.

Moon returned to school in 1980 only to be arrested again.

His close friendship with future president Roh began in 1982 when they opened a law firm in Busan focusing on human and civil rights issues.

Both became leading figures in the pro-democracy protests that swept the country in 1987 and led to South Korea's first direct presidential elections the same year.

When Roh entered politics, Moon continued with his legal practice in Busan, defending students and workers arrested for leading protests and labour strikes.

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.

"I was always happy due to the fact that I was able to help others with what I had been trained to do," Moon said in his autobiography.

- Deeper rifts -

The 64-year-old has promised to curb the concentration of economic power in the hands of the chaebols, the family-oriented business groups whose ties to government have been exposed in the wide-ranging scandal that saw Park impeached.

But his opponents say he is narrow-minded and surrounded by jealous loyalists, whose strong factionalism has contributed to the main opposition party splitting.

"When he becomes president, the rift between liberals and conservatives will deepen all the more, and national reconciliation would be further off," former Yonsei University political science professor Kim Syng-Ho said.

Conservative critics also accuse him of being too soft towards nuclear-armed North Korea.

Tensions between Washington and Pyongyang over the latter's weapons and missile programmes have risen in recent weeks, but Moon advocates dialogue and reconciliation with the North to defuse the situation and eventually lure it to negotiations.

He has shown ambivalence over the US missile defence system THAAD, which has been deployed in the South to the fury of China.

That could all lay the ground for a difficult relationship with US President Donald Trump -- who has demanded that Seoul pay for the "billion dollar" system.