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."
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.