In a posting on the company blog entitled "Tweets still must flow," the company said that it hasn't yet used this ability, but, "if and when we are required to withhold a Tweet," it will make an effort to inform the user and mark when and why the content has been withheld.

'Different Ideas' about Expression

The company said that, as it continues to grow internationally, it is moving into countries that "have different ideas about the contours of freedom of expression." It cited such examples as a ban on pro-Nazi content in France and Germany, and bans in many countries on content relating to intellectual property piracy.

Twitter has become a key tool in social change movements, such as the Arab Spring uprisings and Occupy Wall Street demonstrations, and the idea that Twitter could selectively ban tweets is drawing protests.

Some users, who are posting messages with the hashtags #TwitterBlackout and #TwitterCensored, have indicated they will not use Twitter on Saturday, in protest of the new policy. One organization, called Demand Progress, has called on Twitter to "keep fighting for an enabling freedom of expression -- not rationalize away totalitarianism as a legitimate 'different idea.' "

The company said that, in an effort to expand its transparency about the withholding of tweets, it is expanding its partnership with an organization called Chilling Effects, on whose Web site it will post such things as Cease and Desist notices requiring Twitter to remove content relating to intellectual property piracy.

'A Business Decision'

Chilling Effects is a joint project of the Electronic Frontier Foundation and several universities, including Harvard, Stanford, University of California at Berkeley, George Washington School of Law, and others. The site said that the organization is designed to inform users and site owners about their online rights, their legal liabilities for postings on their sites, and similar information.

A key issue that has some Twitter-watchers concerned is that the company is not clear in what cases it might censor tweets, beyond the narrow legal requirements related to intellectual property issues or a ban on Nazi references in some countries.

Thursday's announcement referenced a posting by the company a year ago, entitled "The Tweets Must Flow." In that posting, the company took the position that it removes "illegal Tweets and spam," but noted that it strives to keep the rules for such removals very narrow, and that it strives "not to remove Tweets on the basis of their content." The new policy does not reiterate the company's efforts to keep the rules very narrow.

Brad Shimmin, an analyst with Current Analysis, said that the move appeared to be "a business decision that Twitter may have to make" in order to operate on a global basis.

However, he noted, there are a variety of possible ramifications. For instance, Twitter may get embroiled in broad-based, SOPA-like efforts to counter intellectual property infringement. The policy also gives the company the ability to conduct censorship in order to incur favor with a government, and it raises the possibility that Twitter might become a series of country-delineated "walled gardens."