The front-runner to succeed South Korea's impeached president Park Geun-Hye after her dismissal over a corruption scandal is a former special forces soldier, pro-democracy activist and human rights lawyer.

The irony is that Moon Jae-In of the Democratic Party was once chief of staff to left-leaning president Roh Moo-Hyun, who committed suicide in 2009 after being questioned over graft allegations.

Even so, Roh invariably polls as South Korea's most beloved ex-president, with other former heads of state generally considered much more corrupt.

South Korea must hold a presidential election within 60 days and Moon, a former Democratic Party leader narrowly beaten by Park in 2012, has long been leading the polls.

A Realmeter survey this week put him on 36.1 percent, with his nearest rival, acting president Hwang Kyo-Ahn trailing far behind at 14.2 percent.

"He is a liberal champion with good chances of winning the next presidential election," Park Kie-Duck, former head of the private Sejong Institute told AFP.

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 the analyst sounded a warning: "He lacks political acumen," he said. "He is too soft to cope with the dirty games in realpolitik."

According to conservative critics, Moon is also too soft towards North Korea.

Nuclear-armed Pyongyang launched a flight of four missiles towards Japan this week in what it said was a drill for an attack on US bases in the country.

But in December Moon said that if elected, he was willing to visit North Korea ahead of the United States, the South's security guarantor.

Facing criticism, he said he meant defusing tensions with the North was an issue of utmost urgency.

A US missile defence system, THAAD, is being deployed to the South in the face of threats from the North, infuriating Beijing, which has imposed a series of measures seen as economic retaliation.

Moon was ambivalent about the issue this week, saying it needed to be carefully handled as it would bring the South "both gains and losses".

But his military service as a paratrooper adds to his credentials.

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

His father was a menial worker at a prisoner-of-war camp in Geoje 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.

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

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.

Despite the scandal that led to his mentor killing himself by jumping off a cliff, said Kang Won-Taek, a political science professor at Seoul National University, Moon "always remained untainted by corruption allegations and abuse of power." (AP)