Main opposition presidential nominee Yoon Suk-yeol stressed Friday that his investigation of former President Park Geun-hye was carried out in the line of duty as a senior prosecutor amid speculation her release from prison could hurt his election chances.

Park became a free woman at midnight under a presidential pardon after serving nearly five years in prison. Her release puts Yoon of the People Power Party (PPP) in an awkward position as he headed the investigation of Park's corruption scandal that eventually led to her impeachment, ouster from office and imprisonment in 2017.

In a book that was published on Thursday, which was based on Park's letters from prison, Park did not directly mention Yoon, but claimed that an additional arrest warrant issued on her in October 2017 was unreasonable. Yoon was head of the Seoul Central District Office at the time.

"I have not read the book yet, but now I am a politician," Yoon said of Park's claim in the book. "I had to enforce laws as a public servant, but now as a politician, I am in a position to think about things like the national unity and evaluation of those who made big contributions to the country."

Park's pardon is widely expected to have a big impact on the March 9 presidential election as she has commanded strong support of voters in Daegu and North Gyeongsang Province, a stronghold of the PPP.

In recent days, Yoon has been tying to sort out their troubled past relationship to consolidate conservative voters for the presidential election.

Analysts said Park's support for Yoon may bring the conservative bloc together, but also could turn away moderate supporters.

Yoon earlier said he is sorry for the corruption probe on Park.

"I just wish for Park to make a quick recovery of her health," he said. (Yonhap)