ISTANBUL -- Thousands of Turks gathered in some 40 cities and towns around the country on Sunday, to join marches organized on Facebook against state Internet censorship.

The trigger for the protests was a decision by Turkey's Internet regulator, the Information and Communication Technologies Authority, or BTK, to introduce a selection of filters that Turkish Internet users would choose from before browsing the Internet, beginning in August.

"These filters would turn the Internet into a state-controlled area," said Serkan Dogan, 29, an Istanbul software programmer who said he was taking part in his first political demonstration. "You'd enter a channel leading you to the server of the state, which distributes the Internet to millions of users. The system enables the control of citizens…like telephone tapping."

Protesters also have been upset by a BTK announcement that it will ban a list of words from use in domain names. According to press reports, those banned words would include things as common as "blonde" and "sister-in-law."

Earlier this month, BTK head Tayfun Acarer told reporters that the accusations of censorship were baseless, as use of the four proposed state filters -- children's profile, family profile, domestic or standard profile -- would be voluntary. However, Mr. Acarer gave few details of how the system would work.