China has stepped up monitoring of dissidents and Internet censorship ahead of June 4, when hundreds were killed in 1989 after Beijing sent soldiers to its central Tiananmen Square to disperse protestors. The authoritarian government wants to ensure that date and other sensitive anniversaries this year pass without public disturbances, observers say. In recent months, China has blocked YouTube and closed two blog hosting sites, bullog.cn and fatianxia.com, known for their liberal content.

Those moves added to an existing set of measures China uses to control online activity. China blocks access to countless Web sites as part of a filtering system critics call the "Great Firewall," including home pages of human rights advocacy groups, parts of Wikipedia and some foreign news sites. Government censors patrol online forums for pornographic or politically subversive content, which Web site managers often delete themselves to avoid punishment by authorities.

But the challenge to censor speech effectively has become greater as China's Internet population expands. China had almost 300 million Internet users at the end of last year -- a thousand-fold increase over just the past 12 years, according to China's domain registration agency.

The government has appeared recently to be slightly more tolerant of some types of speech. The rise of blogs and online forums, impossible for Beijing to fully control, has given people a direct and far-reaching way to air grievances. The authorities have also seemed to yield on occasion to online public opinion. This month, a blogger who had been detained for writing about corrupt village elections had charges against him dropped after he continued posting about the poll online, pushing himself into the public eye.

But while disgraced local officials are often fair targets for complaints, criticism of the ruling Communist Party itself, or of systemic problems at all, remains largely off-limits both online and offline.