A veteran South Korean opposition lawmaker slammed prosecutors there on Tuesday for indicting a Japanese reporter who allegedly defamed President Park Geun-hye by repeating innuendo about her actions in the hours following the deadly Sewol ferry sinking.

"It's seriously wrong for the prosecution to launch a probe and indict (a journalist) on charges of defamation just because what has been published was not true," Moon Jae-in of the main opposition New Politics Alliance for Democracy told reporters.

The indictment contravenes global standards on freedom of the press and has become "shameful" internationally, he added.

The Seoul Central District Court will hold preliminary hearings on Thursday concerning the defamation charges against Tatsuya Kato, the former Seoul bureau chief of Sankei Shimbun, and will seek his plea.

In an article published online on Aug. 3, Kato suggested Park was not at her office for seven hours on April 16, the day 300 people died when the Sewol ferry sank. He repeated unconfirmed allegations that she may have been engaged at that time in a secret rendezvous with a recently divorced former aide.

Kato cited hearsay among stockbrokers and quoted from the Chosun Ilbo, a major South Korean newspaper. The newspaper was the first to report the rumor but none of its staff have been charged with defamation.

A conservative South Korean civic group filed a defamation suit against the Sankei bureau on Aug. 9 over the article, prompting the prosecutors to open an investigation.

The indictment "is not in line with legal principles and precedents, and global standards concerning freedom of the press," said Moon, who lost to Park in the presidential election in 2012.