BEIJING: China's campaign against online rumors, which critics say is crushing free speech, has been highly successful in "cleaning" the Internet, a top official of the country's internet regulator said on Thursday.

China has the world's most sophisticated online censorship system, known outside the country as the Great Firewall. It blocks many social media websites, such as Twitter, Facebook, YouTube and others, along with many sites hosted in Taiwan and those of rights groups and some foreign media agencies.

The crackdown on online rumors is really intended to quash anti-government discourse, activists say. High profile users of Sina Weibo, China's Twitter-like microblog, have been targeted, apparently for political speech.

In a rare public appearance, Ren Xianliang, vice minister of the State Internet Information Office, emphasized China's commitment to scrubbing the web of content it deemed critical or offensive.

"The fight against rumors has received a positive response and has been quite effective," he said.

"The Internet has become clean. The frequency of slander has declined, but it has not impacted the orderly flow of information."

Although social media has become a platform for users to voice complaints and criticism about the government, authorities force domestic internet firms to delete user-posted content they consider too politically sensitive.

China will work to strengthen regulation of the internet by training local internet regulators and net companies, Ren added, and further "manage" search and microblogs as well as Tencent's popular WeChat app.

"We will meet the demands of the people to create a cyberspace with Chinese characteristics," Ren said.

He reiterated China's right to block websites with information on Tibetan independence or support for separatists in China's far western region of Xinjiang.

"Some websites propagating material on Tibet and Xinjiang aim to split our nation, or try to subvert the power of the state," Ren added. "This runs counter to China's laws and regulations."