The No. 1 opposition Democratic Party called Saturday for a thorough investigation into allegations that the nation's chief prosecutor was forced to resign under pressure from the presidential office over a personal matter.

On Friday, Prosecutor-General Chae Dong-wook stepped down amid allegations that he has a son born from an extramarital affair. 

Chae strongly denied the allegations raised by a local newspaper but resigned hours after Justice Minister Hwang Kyo-ahn ordered an internal investigation into the alleged scandal, saying that he could no longer do his job properly under the circumstances.

It was the first time that a justice minister has ordered an inspection into the country's top prosecutor over a personal matter.

Government critics suspect that Chae was forced to resign because of his uncompromising stance with an ongoing prosecution investigation into allegations that the National Intelligence Service (NIS) meddled in last year's presidential election by waging an online smear campaign against the opposition candidate.

"The presidential office is presumed to be involved in the allegations, and tried to cover the intelligence agency's involvement in last year's presidential election," the opposition party said in a statement.

The main opposition claimed that Chae's resignation is the result of an attempt by the presidential office and the intelligence agency to tame the prosecution as "a slave to power."

The opposition party demanded that the parliament's judiciary committee convene a meeting to discuss the issue, claiming that the justice minister's inspection order is an attempt to remove Chae from office and affect the trial on the election-meddling scandal.

Many former top prosecutors have ended their careers in disgrace after being held responsible for personal and other reasons. Chae's immediate predecessor Han Sang-dae resigned earlier this year over a string of high-profile scandals within the organization. (Yonhap news)