South Korean presidential front-runner Moon Jae-in has outraged persecuted sexual minority groups by saying during a television debate that he opposes homosexuality, something his supporters say he had to do to win office in the deeply conservative country.

Moon, a liberal candidate and former human rights lawyer, made the comments Tuesday night while responding to a conservative presidential candidate who argued gay soldiers were weakening the country's military.

Gay rights advocates, some of whom who were dragged away after approaching Moon following a speech in Seoul on Wednesday, see Moon's words as hate speech.

Moon's supporters defended his comments as in line with many South Koreans' views. Moon's camp didn't immediately respond to the criticism by gay rights activists.

Sexual minorities are harshly stigmatized in South Korea and struggle with political visibility because a powerful Christian lobby keeps politicians from passing anti-discrimination laws. Among the five candidates who participated in the debate, only Sim Sang-jung, representing the minor Justice Party, openly advocates gay rights.

South Korean presidential candidate Moon Jae-in is coming under criticism for his comments opposing homosexuality, given his past as a human rights lawyer and the fact he represents the liberal Democratic Party. (Kim Hong-Ji/AFP/Getty Images)

After the conservative candidate, Hong Joon-pyo, argued that gay soldiers were weakening the country's military, Moon said he agreed.

Hong then asked Moon whether he opposed homosexuality, to which Moon replied, "I oppose." When Hong double-checked, Moon said, "Of course."

Moon favoured in May 9 election

Hong represents the party of ousted President Park Geun-hye, who was arrested last month and now faces the prospect of a prison term over corruption charges.

He recently angered female voters over revelations in a 2005 memoir that he assisted a friend in a failed date rape attempt as a teen. The incident involved mixing the woman's drink with a livestock stimulant, Hong wrote.

A watchdog has recently accused South Korea's military of hunting down and prosecuting gay servicemen after a video of two men was posted on the internet. That raised worries of more discrimination and hate crimes against sexual minorities.

Moon later in the debate said that he opposed "discrimination based on homosexuality," but was also against legalizing same-sex marriage.

"Moon needs to offer an apology and a correction of his comments made on live television," said Jung Yol, a gay rights activist. "What he said was clearly hate speech, and since he is the candidate favoured to win the election, his words can influence how people think."

South Korea's constitutional Court formally removed Park from office on March 10. That triggered a 60-day presidential race. Opinion surveys favour Moon to win the May 9 election.