The Internet has brought new hope to reformists in China since the country crushed pro-democracy protests in the capital 20 years ago. But as dissidents have gone high-tech, the government in turn has worked to restrict free speech on the Internet, stifling threats to its rule that could grow online.
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.
Besides Tiananmen, this year is the 10th anniversary of China banning Falun Gong, a spiritual movement, and the 60th anniversary of China's founding, which Beijing will mark with a big military parade.
Dissidents, including many under house arrest and constant monitoring, have turned to tools like proxy servers and Skype to communicate with each other and the world outside China.
But while Skype encrypts calls and instant messages, the only version available on the Chinese Internet comes from a joint venture between Skype and a Chinese portal. That version uses keyword filtering to block messages with sensitive content, which it then stores along with user data, researchers at the University of Toronto said in a report last year.
China this year also redoubled its efforts against at least one popular program used to circumvent its Internet filtering. Chinese users of the program, called FreeGate, began reporting problems including slower loading of foreign Web sites early this year, said Bill Xia, president of Dynamic Internet Technology, the developer of the software.
Hundreds of thousands of people use FreeGate each day, including many dissidents, said Xia. The program encrypts users' communication and routes it through IP (Internet Protocol) addresses abroad, granting access to Web sites blocked in China.
Chinese censors have long tried to identify encrypted FreeGate traffic so they can block the foreign IP addresses channeling it, said Xia. Users are given a new IP address when that happens, but this year China's IP blocking became faster and more aggressive, Xia said.
Beijing is nervous about citizens finding sensitive information online, especially this year, Xia said.
China may have boosted its manpower and improved integration with the country's international IP routers to expand the blocking against FreeGate, Xia said. Xia's team has since expanded its network of IPs to make the program's traffic harder to identify, and its speed has since returned to normal, he said.
That followed years of technological back-and-forth with China's censors, with developers of FreeGate always upgrading it to counter China's most recent blocking tactics, Xia said.
Mobile phones have helped information in China flow more freely as well. Videos captured on cell phones of riots last year in Tibet reached a global audience when they were posted online.
Tibet is controlled by China, but many residents still revere the Dalai Lama, the leader of Tibetan Buddhism who has lived in exile from the region for decades.
Mass text messaging on mobile phones drew thousands of Chinese in Xiamen, a coastal city, to demonstrate against the construction of a chemical plant two years ago.
But China also runs a filtering system for text messages that contain political keywords, and authorities often harass or detain individuals who lead demonstrations. Phones used by known dissidents are usually tapped.
Rights groups have long used their Web sites and mailing lists to prolong the memory of the Tiananmen crackdown. But with the anniversary approaching next week, efforts at commemoration in China appear largely absent, both on the Internet and on the ground.