Eight members of the Chinese media have been detained by police for an alleged scam in which a major business news website and two public relations firms collaborated to extort money from companies in return for favorable coverage on the site.

The suspects are from news website 21cbh.com, a PR firm based in Shanghai and another based in the southern metropolis of Shenzhen, the Shanghai Municipal Public Security Office told Xinhua on Thursday.

They include the editor-in-chief of 21cbh.com, who is surnamed Liu, the deputy editor-in-chief, who is surnamed Zhou, 21cbh.com reporters and employees of its marketing department, as well as heads of the two PR firms. Together, they extorted money from dozens of companies since November 2013, said police.

The list of victims covers many listed companies or famous enterprises from Beijing, Shanghai and Guangdong that are planning to go public, restructuring or making a business transition.

Having identified these companies as particularly susceptible to media coverage, the suspects targeted them in the name of news reporting. After the companies handed over "huge payments," 21cbh.com released positive stories with exaggerated content about them, according to the office.

The website journalists are also suspected of helping to rebut or conceal negative news reports on companies that paid up.

The police didn't specify the amount the suspects allegedly charged for these services, but said they have made "huge personal profits" from their actions.

For companies which declined the suspects' solicitation, 21cbh.com published "malicious attacks" on them or the suspects demanded money to ensure the negative stories did not see the light of the day, said police.

They received reports on the alleged scam from individuals and companies, detaining the eight suspects in a raid on Wednesday.

Further investigation into the case is under way.

Under regulations set by the State Administration of Press, Publication, Radio, Film and Television, which manages the practices of all journalists in China, press credentials will be revoked in cases of media organizations acting illegally.

21cbh.com is run by Guangdong-based 21st Century Media Co., Ltd., which claims on its website to be "the largest professional media operator in the Chinese financial and business media industry."

The company's publications include 21st Century Business Herald, Money Week and 21st Century Business Review.

21cbh.com has offices in Shanghai, Beijing and Guangzhou.

Chinese authorities have been stepping up a crackdown on extortion in the media and paid-for news.

They have discovered problems including press cards being issued to people who are not journalists, and newspaper websites being contracted to advertisement or PR agencies.