The restrictions imposed by the government made such social-networking sites as Twitter and Flickr more prominent - with even the U.S. State Department calling on Twitter to put off a scheduled shutdown for maintenance.

Iranians were posting items online, but it wasn't known how much of that information was being seen by others inside the country. And although some of the posts on Twitter appeared to be from users in Tehran, others clearly were not.

Following a massive opposition rally Monday, authorities restricted journalists - including Iranians working for foreign media - from reporting on the streets. They could effectively only work from their offices, conducting telephone interviews and monitoring official sources such as state TV.

Some foreign journalists were forced to leave Iran because the government wouldn't extend the visas they received to cover Friday's election, in which President Mahmoud Ahmadinejad was declared the landslide winner.

"Clearly, when our journalists can't go out and see things and talk to people, our ability to tell the story is not as good as when we are able to go out to report and take pictures and video," AP Executive Editor Kathleen Carroll said.

When controls are imposed, "we work with those restrictions, keeping in mind our ultimate goal is to be able to do our jobs as journalists," she said.

The London-based Reuters news agency included an editor's note on its stories out of Iran saying its coverage was subject to a ban on "foreign media leaving the office to report, film, or take pictures in Tehran."

CNN turned in part to the social-networking sites, broadcasting images posted on Facebook and Twitter, and explaining on-air that it was using "creativity" to cover a big event under government restrictions.

"We cannot verify readily some of this material that we're going to show you," correspondent David Mattingly warned viewers. Much of the material on Twitter is posted anonymously.

CNN spokeswoman Bridget Leininger said that adding context and explaining issues was necessary when reporting with such online sources. "We are committed to making the most information available in a tough news environment, while being totally transparent with the audience," she said.

Iranian journalists have also been targeted, including at least 10 who have been arrested, according to the press freedom group Reporters Without Borders.

"We are very worried about them," said Jean-Francois Julliard, secretary-general of the Paris-based organization. "We don't know where they have been detained, in what conditions. And we know as well that bloggers have been arrested, people who just took pictures with their mobile phones have been arrested, and all journalists are under threat."

The Iranian government also tried to stop its citizens from spreading information. Internet service and cell phone service was intermittent Tuesday, with long delays getting online. Many sites, including some that support reformist challenger Mir Hossein Mousavi, were blocked.

Even under ideal circumstances, only a quarter of Iran's 70 million people have Internet access at home or at work, and Internet cafes are found only in major cities.

Fearing Iranian government attempts to track Twitter users, some of those abroad changed their settings to make it appear they're in Iran - hoping to make it more difficult for authorities to find Iranian users.

Users of Twitter have also been sharing ways, called proxies, that Iranians can use to circumvent the efforts to block sites.

The importance of Twitter in Iran has been recognized by the U.S. State Department, which contacted the company during the weekend to request that Twitter not take its service down for scheduled maintenance, according to an official who spoke on condition of anonymity because of the sensitivity of the situation.

San Francisco-based Twitter Inc. delayed the planned 90-minute shutdown, citing the role Twitter was playing in Iran.

To stop citizens from getting out their text messages, tweets, photos and e-mails, Iran would have to restrict Web access entirely, following the footsteps of North Korea or Cuba, said John Palfrey, an Internet censorship expert at Harvard University.

Reporters were also restricted during the 1979 Iranian revolution, which saw the installation of the Islamic regime in power today.

Back then, reporters relied on landlines and Telex services of the government telecommunications company to get out the news.

Instead of relaying copy from American news organizations that were perceived as biased in favor of the monarchy, revolutionary sympathizers in the government would often block the Americans' circuits.

Government censors and the Internet have often clashed.

This April, protesters in Moldova used Twitter and the Internet when mobile phones and cable news television stations went down.

Myanmar's military government has cracked down on Internet use by dissident groups, temporarily shutting down international connections and jailing bloggers.

"No one quite knows what sort of pressure ... would actually lead to a free election," said Ethan Zuckerman, research fellow at Harvard University's Berkman Center for Internet and Society. "Certainly, international attention makes it harder to wash things under the rug."

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Ortutay contributed from New York. Associated Press writers Matthew Lee in Washington, Robert H. Reid in Baghdad, Audrey Horowitz in Paris and Andrew Vanacore in New York also contributed to this report.

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