Access to the web in China has been restricted from the start to state-run telephone networks. The Public Security Bureau has used legal and technical means to throw up roadblocks at all the main gateways to restrict the entry of all content deemed unsuitable for Chinese surfers.

Logging on to the BBC website in China is next to impossible. Such a restriction involves only a block on that site. Yet China’s net regulators are far more sophisticated and effective: they use controls to bar access via web pages, online chat rooms and even e-mail to a wide array of sensitive material, from pornography to religion. The police do their best to monitor all of these, although cannier users turn to “proxy servers” to carry messages that hide banned words.

Search providers, such as Microsoft, set up their own filters as required by the authorities to block the use of certain keywords, such as “democracy”, “human rights” and “Tibet independence”. Most sensitive of all is “Falungong”, the name of a quasi-religious sect whose ability to mobilise and organise the public en masse sends jitters through Communist Party rulers. An attempt to search for the sect via Google results in the entire search engine shutting down for 20 minutes. Other sites that are inaccessible by normal means include Amnesty International.

Saudi Arabia has some of the strictest controls. It makes all traffic flow through a central agency where it can be monitored. But the authorities are faced with constant challenges from new technologies, such as Bluetooth.

Iran’s filtering system is “one of the world’s most substantial censorship regimes,” according to The OpenNet Initiative, a partnership of researchers in the US, Britain and Canada. Thirty-four per cent of more than 1,000 web addresses they tested were blocked. These included pornographic sites, “anonymiser tools”, sites with gay and lesbian content, some politically sensitive sites and weblogs.

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