Google's services are blocked or censored in a quarter of the countries it operates in.

"China is the most polarising example, but it is not the only one. Google products - from search and Blogger to YouTube and Google Docs - have been blocked in 25 of the 100 countries where we offer our services," Rachel Whetstone, Google's vice president of global communications and public affairs, said on the company's blog.

"In addition, we regularly receive government requests to restrict or remove content from our properties," she said, adding that Google has argued to narrow the scope of such requests when it believes the request is "overly broad."

Google made headlines earlier this year, when executives announced plans to shut down a censored version of its search engine that it developed for the Chinese market.

After failing to reach a compromise over censorship with the Chinese government, Google closed the censored search engine in March, redirecting traffic to an uncensored search engine in Hong Kong

China has long had one of the most sophisticated programs designed to control internet access for local users.

Chinese laws require internet companies that operate there to block or remove objectionable content, which includes pornography as well as information relating to human rights and other topics.

But China isn't the only country that wants to manage internet content.

For example, Australia is proceeding with plans to block access to internet sites with material on sexual abuse of children and information that could be used to commit crimes, after tests showed blocking access to blacklisted sites didn't slow down internet access for users.

Opponents have objected to the Australian plan, noting that the list of blocked sites includes links that extend beyond the intended scope.

In addition, users are able to circumvent the blacklist to access these sites, just as Chinese users are able to circumvent censorship controls there.

See also: Google password system stolen by 'China attacks'