Chinese authorities have blocked most domestic users from the main Google.com search engine, a media watchdog said.
More than 100 million people are online in China
Internet users in major Chinese cities faced difficulties accessing Google's international site in the past week, Reporters Without Borders said.
But Google.cn, the controversial Chinese language version launched in January, has not been affected.
The site blocks politically sensitive material to comply with government censorship rules.
"It was only to be expected that Google.com would be gradually sidelined after the censored version was launched in January," Reporters Without Borders said in a statement.
"Google has just definitively joined the club of Western companies that comply with online censorship in China," the organisation said.
Google.com, the search engine's uncensored international site, had previously been available to Chinese web users, but problems accessing the site had been reported across the country recently. It was blocked nationwide on 31 May, the statement said.
The blocking was also being extended to Google News and Google Mail, Reporters Without Borders said.
A spokeswoman for Google in Beijing said that the problem was under investigation.
GREAT FIREWALL OF CHINA
Foreign websites covering politics and sensitive issues are blocked
Chinese internet providers face strict censorship
Websites, forums and blogs must officially register and are monitored
China's internet 'police' thought to number 50,000 censors
The spokeswoman, Cui Jin, said she could not give any more information.
On Tuesday, Google co-founder Sergey Brin defended his company's decision to launch the censored Google.cn service, a move which drew heavy criticism.
"We felt that perhaps we could compromise our principles but provide ultimately more information for the Chinese and be a more effective service," he said.
"Perhaps now the principled approach makes more sense."
In addition to Google, US companies Microsoft, Yahoo and Cisco Systems have also been accused of accommodating China's demands on censorship in return for access to its huge internet market.
The Chinese government's internet filtering is some of the most sophisticated in the world.
Content considered to be a threat, including references to the Tiananmen Square massacre and notable dissidents, is blocked.
Chinese authorities have also stepped up measures against software designed to bypass internet censorship, the Reporters Without Borders statement said.