The online collective known as "Anonymous," which has attacked a number of corporate and other websites in apparent retaliation for moves against the document-leaking organization WikiLeaks, said Monday it has turned its attention to Iran.

A strategist for the collective said the group had launched so-called denial of service, or DOS, attacks on a number of high-profile Iranian government sites. In a DOS attack, computers flood a server to prevent it from displaying a web page.

The attacks coincided with mass protests in Tehran on Monday. Opposition leaders there renewed calls for street demonstrations amid wider Mideast unrest, including revolts in Tunisia and Egypt that toppled long-time rulers in those countries.

Barrett Brown, who describes himself as a strategist for Anonymous, said the group targeted the websites of Iran's state news broadcaster and the website of President Mahmoud Ahmadinejad, among other sites. It is unclear how successful the attacks were, but those two sites weren't accessible late Monday.

"Iran was targeted pursuant to our strategy of putting our time and energy towards those countries that are best able to make use of our assistance," Mr. Brown said, adding: "The people of Iran have already made clear their intent to revolt, which is a major factor in our decisions regarding who to support and when."

Anonymous currently is the focus of an international probe by authorities in the U.S., U.K., the Netherlands, Sweden and Germany. Late last year, a number of organizations and individuals suffered DOS attacks. The attacks appeared to have been an effort by hackers to exact revenge on behalf of WikiLeaks.

Victims included MasterCard Inc. and Visa Europe, both of which had suspended payments to WikiLeaks. The payment providers acted amid a showdown between the U.S. government and Wikileaks over the website's release of thousands of classified U.S. documents.

Anonymous is a loose-knit collective, without a centralized hierarchy or chief but with a core group of about a dozen individuals. Members communicate via Internet chat rooms and social networking sites like Twitter.

Evan Perez in Washington contributed to this article.

Write to Cassell Bryan-Low at cassell.bryan-low@wsj.com