But other terms or Web sites are suddenly or sporadically blocked for reasons no ordinary user can fathom. One Beijing technology consultant, who asked not to be identified for fear of retribution against his company, said that for several days last week he could not visit the Web site for the Hong Kong Stock Exchange without a proxy. LinkedIn, a networking platform, was blocked for a day during the height of government concerns over Internet-based calls for protests in Chinese cities a few weeks ago, he said.

Hu Yong, a media professor at Peking University, said government censors were constantly spotting and reacting to new perceived threats. “The technology is improving and the range of sensitive terms is expanding because the depth and breadth of things they must manage just keeps on growing,” Mr. Hu said.

China’s censorship machine has been operating ever more efficiently since mid-2008, and restrictions once viewed as temporary — like bans on Facebook, YouTube and Twitter — are now considered permanent. Government-friendly alternatives have sprung and developed a following.

Few analysts believe that the government will loosen controls any time soon, with events it considers politically sensitive swamping the calendar, including a turnover in the Communist Party’s top leadership next year.

“It has been double the guard, and double the guard, and you never hear proclamations about things being relaxed,” said Duncan Clark, chairman of BDA China, an investment and strategy consultancy based in Beijing, and a 17-year resident of China. “We have never seen this level of control in the time I have been here, and I have been here since the beginning of the Internet.”

How far China will clamp down on electronic communications is unclear. “There’s a lot more they can do, but they’ve been holding back,” said Bill Bishop, a Internet expert based in Beijing. Some analysts suggest that officials are exploring just how much inconvenience the Chinese are willing to tolerate. While sentiment is hard to gauge, a certain segment of society rejects censorship.

For many users, an inoperable V.P.N. is an inconvenience, not a crisis. But Internet consultants said interfering with an e-mail service on which people depend every day is more serious. “How people respond is going to be more intense, more visceral,” one consultant said.