Sina.com and Sohu.com impose strict censorship

"We have undergone an upgrade of the search functions of our internal system. This morning the search engine returned to normal usage," the Sina's customer representative said.

China has for years been battling to censor the Internet of pornographic and violent content, as well as political and religious material that it believes could spark social unrest.

Internet portals in China must censor their sites for "unhealthy content".

But expect for pornography and strident anti-government opposition, "unhealthy content" has been difficult to define. This has led to the unintended blocking of large swathes of healthy information, critics have said.

"Unhealthy content" increasingly appears to cover politically sensitive material.

Searches on terms such as the banned religious group Falungong, the Dalai Lama, Tibet or Taiwan Independence typically yield results only favorable to the government's position on the issues.

Western Internet firms such as Microsoft, Google and Yahoo, which have self-censored their content in China, have been criticized by human rights groups.

China has roughly 111 million Internet users, with the majority being younger users.