The prime target at the outset was Hong Joon-pyo of the conservative Liberty Korea Party whose past confession of his role in a friend's attempted rape 45 years ago got fresh attention ahead of the May 9 election.

Sim Sang-jeung, the nominee of the minor progressive Justice Party, opened fire by declaring she would not engage in a debate with Hong.

"I cannot recognize a candidate who was an accomplice in a sex crime as a rival candidate," she said. "It would be natural for Hong to resign in consideration of the shame felt by the people and national prestige."

Hong, a former prosecutor and provincial governor, wrote in an autobiographical essay in 2005 that he provided an animal aphrodisiac powder to a friend who wanted to rape a coed while he was a law student at Seoul's Korea University.

The candidate apologized again, saying he feels responsible for not stopping his friend. But he also expressed frustration that the issue was being brought up again even after his "confession."

Front-runner Moon Jae-in of the Democratic Party was pressured to clarify allegations the government of then-President Roh Moo-hyun consulted North Korea before abstaining from a UN vote on a North Korean human rights resolution in 2007. Moon served as Roh's chief of staff at the time.

Moon reaffirmed the Roh administration decided to abstain before notifying the North of the decision.

Yoo Seong-min of the splinter conservative Bareun Party asked Moon whether he would quit the race if his remarks turn out to be a lie.

Earlier in the day, Moon's campaign unveiled former presidential aides' memos to counter the claim. The documents indicated Roh had decided on Nov. 16 that year not to join the vote in consideration of improving inter-Korean ties, and notified Pyongyang of the decision on Nov. 19.

But Song Min-soon, the then-foreign minister who first raised the allegation in his memoir in October, countered that the decision was only provisional and the discussion continued until Nov. 20 just before the vote was conducted the following day.

Ahn Cheol-soo of the center-left People's Party continued to skewer Moon in what appeared to be an aggressive push to beat his rival in their second matchup in five years.

The two candidates competed for liberal votes in the 2012 presidential race before Ahn dropped out in support of Moon.

Ahn, who is now the runner-up in opinion surveys, criticized Moon for employing negative tactics including branding him as an "avatar of MB."

MB is the initials of former conservative President Lee Myung-bak who was in office from 2008-2013.

"In the previous election, I yielded my candidacy. That was because I was determined that the Lee Myung-bak administration should not be extended," Ahn said. "(You) told me something similar back then as well. Am I still an avatar of MB?"'

Moon dodged answering directly, saying he had heard such talk but had nothing to say in response to rumors.

"If it's not true, explain why it isn't true," he said. "Don't look toward me but look toward the people. Are you in politics (simply) to oppose me?"

He later said that he doesn't agree with the "MB avatar" accusation.

Hong chimed in, scolding them for quarreling over petty issues.

"Watching the two of you, I can't tell if it's an emotional fight between elementary school children or a presidential debate," he derided.

The election watchdog is scheduled to hold two more debates on economic and social issues before the election. (Yonhap)