Conservative presidential candidate Hong Joon-pyo apologized during Sunday's TV debate over controversy about his suspected involvement in a potential sex crime about four decades ago.

Hong of the Liberty Korea Party initially wrote about his role in the incident that took place while he was in university in an autobiographical essay published in 2005. The scandal has drawn fresh attention in the run-up to the May 9 presidential election.

"I confessed my part in the incident 12 years ago from a guilty conscience for failing to prevent my friend from attempting a sexual assault," he said.

"Though it was not me who did it, I feel sorry for not preventing it," he added.

During the debate, his presidential rivals demanded he withdraw his candidacy over the incident.

In his memoirs, Hong confessed that he provided an animal-based aphrodisiac powder to his friend who wanted to rape a coed while attending Korea University's Law College in Seoul.

Excerpts of Hong's essay began to spread online last week, sparking public uproar and calls for him to drop out from the presidential race. (Yonhap)