South Korean presidential front-runner Moon Jae-in has outraged sexual minorities by saying that he "opposes" homosexuality, as the deeply conservative country heads toward elections on May 9.

Key points: Moon Jae-in said he agreed that gay soldiers weakened South Korea's military

Moon Jae-in said he agreed that gay soldiers weakened South Korea's military South Korea's military has been accused of hunting down gay servicemen

South Korea's military has been accused of hunting down gay servicemen Sexual minorities are harshly stigmatised in South Korea

Mr Moon, a liberal candidate and former human rights lawyer, made the comments during a televised debate, in response to questions from a conservative presidential candidate who argued gay soldiers were weakening the country's military.

A human rights watchdog recently accused South Korea's military of hunting down and prosecuting gay servicemen, after a sex video between two male soldiers appeared on the internet earlier this year.

South Korea's army says it's conducting a proper criminal investigation on soldiers allegedly involved with filming and uploading the video — but the Military Human Rights Centre for Korea says more than 30 soldiers were being investigated, and at least one has been arrested.

Gay rights advocates, some of whom who were dragged away after approaching Mr Moon following a speech at the National Assembly in Seoul, see his words as hate speech.

A protester holding a rainbow flag approaches Moon Jae-in following a speech. ( AP: Ahn Jung-won )

Police said 13 activists were detained at the scene of Mr Moon's speech on suspicion of violating laws that ban protests inside the gates of the legislature.

Mr Moon's supporters defended his comments as in line with many South Koreans' views, although his camp did not immediately respond to the criticism by gay rights activists.

Sexual minorities are harshly stigmatized in South Korea and struggle with political visibility due to a powerful Christian lobby that keeps politicians from passing anti-discrimination laws.

'His words can influence how people think'

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

Mr Hong then asked Mr Moon whether he opposed homosexuality, to which he replied, "I oppose."

Mr 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, Mr Hong wrote.

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

"Moon needs to offer an apology and a correction of his comments made on live television," Jung Yol, a gay rights activist, said.

"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."

Lawyers for a Democratic Society, a group of South Korean human rights lawyers to which Mr Moon once belonged, issued a statement denouncing the comments made by both candidates, saying they revealed their ignorance on human and constitutional rights.

"We cannot look away from the issue as the candidates' comments could encourage hate speech and discrimination against social minorities across the society," the statement said.

AP