"We hope that related authorities in Hunan will present a convincing and justice-based explanation (on the matter)," the official All-China Journalists Association said in a statement.
The New Express, a newspaper based in the southern city of Guangzhou, published a front-page plea for the second day today for police to release its journalist Chen Yongzhou.
Chen was detained on Saturday in Guangzhou by Changsha police on "suspicion of damaging business reputation" after he reported "financial problems" at Zoomlion, a partly state-owned large engineering company based in Changsha, capital of central China's Hunan Province.
Of the 18 reports, 14 were written by Chen.
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The articles were written between September 26, 2012 and August 8 this year.
The New Express denied Zoomlion's accusation, saying Chen's reports were objective.
"We did not discover Chen did anything that was against professional ethics and laws," said a senior executive on condition of anonymity.
According to the Changsha police bureau, it approved the criminal detention of Chen on Saturday in accordance with the law.
Meanwhile, the country's media regulator General Administration of Press and Publication, Radio, Film and Television said it was "highly concerned" by the detention.