WHEN thousands of protestors took to the streets in Iran following this year's disputed presidential election, Twitter messages sent by activists let the world know about the brutal policing that followed. A few months earlier, campaigners in Moldova used Facebook to organise protests against the country's communist government, and elsewhere too the internet is playing an increasing role in political dissent.
Now governments are trying to regain control. By reinforcing their efforts to monitor activity online, they hope to deprive dissenters of information and the ability to communicate.
The latest evidence of these clampdowns comes in a report on the Middle East and north Africa by the OpenNet Initiative (ONI), a collaboration of researchers based in the UK and North America. Among the restrictions it reports are clampdowns on Facebook in Syria and the use of hidden cameras in Saudi Arabia's internet cafes.
Most of these actions are aimed at stifling political debate. "Political filtering is the common denominator," says Helmi Noman of the Berkman Center for Internet and Society in Boston, who compiled the report. "It's the main target."
Noman asked volunteers to check whether roughly 2000 sites covering a range of subjects, including gambling, political news and humour, are accessible in various countries. He also examined government eavesdropping schemes and the actions of local law enforcers.
The survey showed that governments in the Middle East and north Africa routinely block sites that host discussions critical of their policies or that cover human rights issues. Opposition parties' sites are also censored.
At times entire social networking services, including Twitter, are unavailable, and the same goes for the YouTube video-sharing site. Orkut, a social networking site, is offline in the United Arab Emirates. The BBC Persian site cannot be accessed from Iran. The governments' task of blocking internet access is made easier because many countries in the region have only a handful of service providers.
Governments also keep tabs on who is using the internet and what they are viewing. In March, newspapers in Saudi Arabia reported that police had started visiting internet cafes to ensure that owners had installed cameras to monitor users, as the country's law requires. In Jordan, cafe owners have to record their customers' names and monitor the sites they visit.
Noman says that filtering and monitoring have become more widespread as the internet's role in political activity has increased. "More activists are going online and more activists are being created online," he says.
Monitoring has become more widespread as the internet's role in political activity has increased
What's happening in the region is echoed to some extent in most other parts of the world. Online users almost everywhere are subject to some kind of censorship, the ONI says.
"The Middle East is not on a different trend to the rest of the world," says Jillian York, who helps to co-ordinate the ONI from the Berkman Center. "Filtering seems to be increasing everywhere."
China has developed an extensive system of filters which it uses to block access to content about sensitive topics, such as the protests in 1989 in Tiananamen Square, Beijing. Other Asian nations, including Thailand and Vietnam, have taken action against blogs and news sites that host material critical of their leaders. The ONI found few restrictions of this kind in the Americas, with the notable exception of Cuba, where many people are unable to even access the internet.
In western democracies, censorship is aimed mainly at sites offering child pornography or images of extreme violence. Though few people object to this, there is a danger that innocent sites can get caught in the net. A list of blocked sites that appeared on the Wikileaks website in March - and which the Australian government was considering using as the basis for a government-mandated filtering scheme - accidentally included the website of a Queensland dentist and YouTube videos about euthanasia.
Officials responsible for the filtering do not generally discuss details of the technology involved, but western companies are known to have sold filtering software to some foreign governments. Search engines Google and Yahoo have conformed to restrictions demanded by Chinese censors to avoid being blocked themselves. Online security specialists Websense and Secure Computing say their technology has been deployed without their knowing.
Other organisations based in the west are helping people defeat attempts at censorship. In 2007, researchers at the University of Toronto in Canada released software called Psiphon that allows internet users to bypass censorship controls such as those imposed by China on communication from abroad.
The Tor software created by the Electronic Frontier Foundation in San Francisco is designed to disguise a message's origin by bouncing it between several different routers.
Berkman researcher Jonathan Zittrain has developed Herdict, a site that allows users to report sites that they find blocked. Twitter, Facebook and YouTube are high on the list of affected sites on reports from around the world. The country from where the largest number of reports originates is China, with around 7500 complaints.