The U.S.-based Web site, which has become popular among Internet surfers in China because of its simplicity and ability to run thorough Chinese-language Web searches, was inaccessible via Chinese servers as early as Saturday, users said.
"It's being blocked out of Beijing," said one industry insider, who follows China's regulation of the Internet closely and used his computer to confirm and pinpoint the block.
The government openly attempts to control Web content in China, where the Internet threatens Communist Party control over the media. It blocks several foreign news sites and frequently forces domestic sites to expunge content deemed unwholesome.
Google was being blocked because searches could bring up links to pornography, content associated with the banned spiritual movement Falun Gong and information deemed harmful to national security, according to an article posted on Web portal NetEase.com.
China's media censors tend to be particularly edgy during politically sensitive times, analysts said, and a Google block may be an attempt to sweep up ahead of the Party congress, which is expected to see sweeping leadership changes. It was the first time the government had blocked access to an Internet search engine, analysts said.
"It's pretty surprising," said Nathan Midler, a senior analyst at International Data Corp in Beijing. "Google, as a search engine, doesn't necessarily conjure up the idea of a Web site worth blocking."
There was no official announcement and officials at the Ministry of Information Industry were unavailable for comment. Google officials in California could not be reached.
In May, China lifted blocks on the overseas Web sites of several Western news agencies, but other news sites and politically sensitive Web sites are still unavailable.
Google has caught on quickly in China, becoming a top choice for searching Chinese sites alongside leading Chinese portals Sina.com and Sohu.com, analysts say.
"It's a very, very popular tool and there's a major hassle factor," said Duncan Clark, head of Beijing-based tech consultancy BDA China.
"But ultimately, in the run up to something like the Party congress, it's 'Batten down the hatches!'," he said.
President Jiang Zemin met Party propaganda chiefs from around the country last week and told them it was critical to create a "good atmosphere" for the congress starting on Nov. 8.
Unlike Sohu and Sina, Google's headquarters are in the United States and it does not yet have operations in China.
"It's not like Sina or Sohu where all they need to do is make a phone call and they can remedy the situation. It doesn't even matter if they make a phone call, Google is not going to do anything as a result of it," IDC's Midler said.
The Google block highlights the risks associated with the Internet business in China, Midler said.
"Internet market growth can be hampered, and will be hampered probably, because of the government influence and the government's concern about controlling content on the Internet," he said. "There's a certain element of risk involved."
Tough new Internet regulations and a self-censorship pledge taken in March by 130 major Web portals, including Yahoo, have drawn fire from critics who say they are sacrificing freedom of expression for business.
Despite the apparent block on Google, Yahoo, which uses Google for searches, was still available in China.