The mainland's most-popular campus chat room, Beijing's Tsinghua University, has been closed to non-student visitors in the latest move to clamp down on the free exchange of ideas on internet forums. Operators of the chat room at the Shuimu Tsinghua website ( www.smth.org ) on Wednesday posted a message saying that against their wishes, non-students would no longer be able to log on. Maintained by tech-savvy budding scientists, the chat room has become renowned for its intellectual debate and social commentary, as well as exchanges on the latest information technology, since its launch in 1996. The operators added that non-student visitors had greatly enriched the chat room. 'Off-campus visitors have been part of the community and we will not forget them,' they said. Many of the chat room's frequent visitors were former students of Tsinghua University. In terms of popularity, the chat room was on a par with Peking University's Yitahutu, which boasted 30,000 users before it was shut in September. The policing of campus-related web sites was stepped up after the Ministry of Information Industry released guidelines on the regulation of non-profit activities on the internet. From Sunday, chat room operators and bloggers will be held liable for any 'objectionable content'. Sources said the Ministry of Education had also weighed in with a circular on strengthening 'political thought' at universities, recommending the internet as a powerful tool in this process. The first stage of the crackdown on the Tsinghua chat room required all users to register under their true identities by Tuesday of this week. Although they were still permitted to log on under an alias, the users' personal details were lodged with the operator. Tsinghua had been chosen as a model for implementing the procedure, the sources said. Analysts noted the Communist Party's Propaganda Department had been beefing up operations at 'Office 1106', which monitors cyberspace for any subversive trends, since last year. Portal operators are now required to submit daily reports on public opinion and social trends, while the print media has been prohibited from using content posted at weblogs. Weblog portals have discouraged their users from alluding to politics and other sensitive topics.