Main opposition presidential candidate Yoon Suk-yeol visited the city of Seongnam on Monday to demand an independent investigation into a corruption scandal tied to a development project overseen by his ruling party rival Lee Jae-myung.

Yoon of the People Power Party (PPP) renewed his demand for the appointment of a special counsel after arriving on the grounds of the apartment complex built in Seongnam, just south of Seoul, when Lee was the city's mayor.

"A special counsel is the only answer to uncovering the truth," he said in a prepared speech. "Only a special counsel that follows the will of the people, not of power, can reveal the bare face of the corrupt cartel."

Lee, the presidential nominee of the Democratic Party, has insisted he had no part in a corrupt scheme that awarded a little known asset management company profits worth 1,000 times its investment in the development project.

The prosecution's investigation into the case has led to the indictments of several key figures, while two people previously involved in the project have been found dead in apparent suicides in the past month.

"The realization of fairness and justice begins with the arrest of the true culprit," Yoon said.

The PPP believes Lee was the mastermind behind the corrupt scheme and has repeatedly called for an independent investigation into the case.

Lee and the DP have agreed in principle but differed on the details, including the scope of the investigation, saying it should also cover alleged irregularities committed by Yoon. (Yonhap)