The floor leader of the main opposition People Power Party (PPP) on Thursday demanded a meeting with President Moon Jae-in following revelations that a state investigative agency checked the phone records of its presidential nominee, the nominee's wife and dozens of party lawmakers.

The Corruption Investigation Office for High-ranking Officials (CIO) has come under fire over revelations that it obtained the phone records of journalists in an alleged attempt to identify the source of leaks to the media.

The scandal has added a new layer after the PPP claimed the CIO also checked the phone logs of its presidential candidate Yoon Suk-yeol, his wife Kim Keon-hee and more than 80 of its 105 lawmakers in the National Assembly.

"I will demand clear measures from President Moon Jae-in regarding the CIO's illegal surveillance and crackdown on the opposition party," Rep. Kim Gi-hyeon, the PPP floor leader, said during a party meeting.

According to the PPP, the CIO checked Yoon's phone logs twice in September and once in October, and those of his wife once in October.

The ruling Democratic Party has claimed the CIO's inquiries were part of the legal process of conducting investigations. It has also said the prosecution checked phone records far more times when Yoon was the prosecutor-general.

Yoon took to Facebook to renew his determination to win the March 9 presidential election in the wake of the scandal.

"Standing up against the ruling 'Moon Jae-myung' forces that surveil even the opposition presidential candidate, I will win the fight for a change of government without fail," he wrote, using a portmanteau of the names of the president and the DP presidential nominee, Lee Jae-myung.

Kim Chong-in, the chief of Yoon's campaign, also criticized Moon's silence on the issue.

"I strongly demand that President Moon Jae-in express his intentions regarding what he thinks of this matter," Kim said at the party meeting.

The CIO was launched in January as part of the Moon administration's drive to reform the prosecution. The office is tasked with investigating corruption among senior public officials and their families.

The PPP has demanded the CIO's chief step down, and called for the office's abolition.

The parliamentary legislation and judiciary committee is scheduled to hold a plenary session later Thursday to question CIO chief Kim Jin-wook over the alleged surveillance. (Yonhap)