SHANGHAI (AFP) - Chinese police have detained eight people including the editor of a well-known business newspaper's website for blackmailing dozens of listed companies over coverage, state media reported Thursday.

Those held in the "huge" extortion case include the editor and deputy editor of the 21st Century Business Herald's site and employees of two public relations firms, the official Xinhua news agency said.

The national business newspaper is part of the Nanfang Media Group, which is owned by the Guangdong provincial government, and is well respected among financial professionals.

Website staff colluded with others to demand payments for positive news from listed firms and prominent companies, while publishing "malicious" attacks on those who refused to cooperate, the report said.

The suspects also forced companies to place adverts or sign cooperation agreements for high fees, it said.

The practice of paying media to run - or not to run - stories is believed to be widespread in China, and authorities announced a nationwide campaign against media extortion earlier this year.

The 21st Century Business Herald website - which has offices in Guangzhou, Shenzhen, Beijing and Shanghai - confirmed that "several" employees were taken away by police on Wednesday for investigation, according to a statement, but gave no details.

The two public relations firms involved in the case are Shanghai-based Roya Investment Services and Shenzhen-based Nukirin.

Shanghai police could not be reached for comment on Thursday.

The 21st Century Business Herald was set up in 2001 as the government sought to make state-run media operate on more commercial terms, especially in less sensitive areas such as business news.