During the opening ceremony of the World Summit on the Information Society in Tunis on Wednesday, delegates from across the globe were amazed when the Swiss president, Samuel Schmid, directly criticised Tunisia for its policy of restricting free speech.

"It is, quite frankly, unacceptable for the United Nations to continue to include among its members states which imprison citizens for the sole reason that they have criticised their government on the internet or in the media," he said. "As far as I'm concerned, it goes without saying that here in Tunis - inside these walls as well as outside - everyone can express themselves freely. It is one of the conditions sine qua non for the success of this international conference."

Zine Ben Ali, the Tunisian president who was sitting next to Mr Schmid on-stage, was not happy. But then he also knew there was no chance that his own citizens would hear or read the criticism. It was later confirmed that as soon as Mr Schmid started his criticism, a commentator at national TV broadcaster simply spoke over him. When the event appeared again on the evening news, Mr Schmid had been edited out.

The real problem however was the internet - the very thing that over 20,000 summit attendees had travelled from all over the world to discuss. How could Tunisia hope to prevent word of the criticism getting out when it would soon appear on dozens of websites? But Tunisia has a solution for that as well: nationwide, unassailable internet filtering.

Tunisia has a population of just under 10 million and according to government figures, 850,000 of them use the internet. However, the majority have little choice but to access the internet through a network of more than 300 internet cafes, called Publinets. Net access is provided by 12 internet service providers, every one of which is obliged to pass all its traffic through the Tunisian Internet agency, ATI.

The ATI observes which websites its citizens are visiting and if it doesn’t like the content, it blocks the relevant page, or often the entire website. Instead of the real page, a fake "404" error message appears in the browser, implying that the site or page doesn’t exist.

A partnership between Cambridge, Harvard and Toronto Universities, called the Open Net Initiative, spent a year reviewing the situation in Tunisia and made its results known yesterday. Of 1,923 websites it felt the Tunisian government might be sensitive to, 187 had simply been removed from the internet - in Tunisia at least.

They include Tunisian human rights organisation LTDH (found at www.ltdh.org), the unauthorised opposition political party CPR and Reporters Sans Frontieres. As well as political websites, blogs, discussion forums, anonymising services and e-mail are also targeted by Tunisian government agencies.

The ATI has been busy over the past few weeks. An alternative "Citizens Summit" highlighting censorship problems in Tunisia had its site similarly removed. Yesterday, reporters from the Swiss news website, Swissinfo.org, confirmed that they too had been added to the list thanks to the coverage of their president’s speech. And if Tunisians start accessing this article you’re reading, it too will disappear from their internet.

The filtering is extra-legal, having no foundation in Tunisian law, but also "focused and effective", the Open Net Initiative reports. On top of that, owners of Publinets are required to monitor their customers’ usage and report on it to the authorities. They also have the right to keep a copy of anything saved by a particular user and to demand a user’s national identity card.

It is not as sophisticated or as far ranging as similar censoring systems in China and Iran, but nonetheless undermines Tunisia’s stated policy of preserving openness of the internet. It also makes for the greatest of ironies about the staging of the WSIS in Tunis.

The filtering software used by the Tunisian government, SmartFilter, comes from the greatest proponent of freedom of information on the internet - the United States. With its headquarters in California and with three European offices, including one in Windsor, Secure Computing says its product "enables organisations to understand and monitor their internet use, while taking effective steps to provide appropriate control over outbound web access".

But this control was invisible to delegates at the this week's summit: the United Nations insisted on an unfiltered internet link to the Kram conference centre, writing it into the Host Country Agreement.

Nonetheless, it has put the UN under pressure to justify its choice of Tunisia as the venue for the World Summit. Kofi Annan, the UN secretary-general, answered the concerns directly at a press conference. "I realise this has been a problem and I have raised it at the highest level with the president," he said. "Sometimes by organising these conferences, you can put a spotlight on problems and that helps push the issue forward."

It has certainly achieved that. Each case of attempted censorship by the Tunisian authorities this week has increased press interest in the issue. Refusal to allow Robert Menard, the head of Reporters Without Frontiers, into the country made world headlines.

The barring of human rights organisations at a meeting in downtown Tunis became a diplomatic incident. And extraordinary tactics yesterday in the Kram centre itself, with Tunisian journalists attempting to take over several press conferences given by the Swiss minister of communications, only led to a pack of international reporters ditching the technologies stories they were writing in order to cover the unfolding drama.

The Tunisian internet agency will have to work night and day if it hopes to block all references to these events. The thing is, they probably will.

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