China's media regulator has vowed to protect "lawful reporting rights", state media said, in a rare official intervention over press freedom after a journalist was detained by police.

Chen Yongzhou, with the New Express tabloid, was held last Friday on "suspicion of damaging business reputation" after he wrote a series of articles on "financial problems" at Zoomlion, a partly state-owned construction machinery manufacturer.

The New Express on Wednesday ran a full-page editorial on its front page to call for Chen's release, a rare example of media defying authorities that drew an outpouring of sympathy and support online and among its press peers.

China's General Administration of Press and Publication, Radio, Film and Television (GAPPRFT) said it was "highly concerned" by Chen's detention, the China Press and Publishing Journal -- which is run by the agency -- reported late Wednesday.

The agency has "coordinated with relevant authorities" to ensure the case was handled "in a just and appropriate manner", the report said.

GAPPRFT is a key part of Beijing's mechanisms to control the media, and issues the credentials all Chinese journalists need to be able to work.

"The GAPPRFT firmly supports the media to carry out normal reporting activities and firmly protects the justified and lawful reporting rights of journalists," the report quoted an unidentified official as saying.

But the official added it opposed any "abuse of reporting rights" and hoped all media outlets would cover the incident in an "objective and rational way".

In his published reports Chen accused Zoomlion of providing fraudulent accounting figures such as inflated profit data, and of a suspicious management buyout that caused "losses to state assets".

Zoomlion is about 20 percent owned by the state, and is listed on the Hong Kong and Shenzhen stock exchanges with a total market capitalisation of more than $8 billion.

In an interview with Chinese news portal Sina, a Zoomlion representative said New Express's allegations were groundless.

"As the public sees it, the journalist looks to be weak when confronting a listed company. But the real weak ones are the tens of thousands of small and medium investors and our employees affected by this matter," Sina quoted the official as saying.

"Their interests should not be ignored. What is wrong with us protecting them?"

Sina also quoted a "senior executive" of the company as saying the Communist Party's feared internal investigative organ, the Central Commission for Discipline Inspection, was also involved.

Chinese media were largely critical of Chen's detention, accusing police of abuse of power.

The liberal Southern Weekly said Thursday on its verified account on Sina Weibo, a Chinese equivalent of Twitter, that police were seeking to create a "terrifying atmosphere".

In January journalists at the newspaper, which like the New Express is based in the southern city of Guangzhou, went on strike and hundreds of their supporters staged demonstrations after an article urging greater respect for constitutional rights was censored by an official.

Chinese authorities have launched a broad crackdown on "online rumours", with a recent rule saying that Internet users could face three years in prison for writing defamatory messages that are then re-posted 500 times.

Paris-based press rights group Reporters Without Borders hailed the New Express move as "courageous" and joined its call for Chen to be freed.

"The government must stop harassing journalists and netizens and must end its 'anti-rumour' campaign, which is a pretext for stifling dissent," it said in a statement.

The state-run Global Times on Thursday sought to play down anger against the government and urged authorities at a higher level to intervene.

"Chinese society will adapt to these cases and learn to consider each case as it stands," it said in an editorial.

"This will quell the current fever for relating everything to the 'inaction' of judicial authorities."