The microblogging service Twitter is increasingly becoming Iranian protesters' lifeline to the outside world, as international news organizations are banned from covering the demonstrations.

As Iran's hard-line regime defends the results of Friday's election, in which President Mahmoud Ahmadinejad was declared the winner in a landslide victory, supporters of opposition candidate Mir Hossein Mousavi took to the streets of Tehran Thursday for a fourth day of protests.

And they are spreading their message with tweets.

"Iran stdnt: state TV & Radio went berserk today, all saying are moosavi's supporters are all criminals & they will stop us at any cost," tweets a blogger by the name of madtownbabe.

"Iran has ordered people to remove all material that could 'create tension' from Twitter," writes a blogger named islandchic.

"Show support for democracy in Iran add green overlay to your Twitter avatar," writes msanusiliman.

The accounts provided by these Twitterers are the chief source of news to refute claims broadcast on Iran's state radio, such as: "Despite wide coverage of unrest, foreign media have not been able to provide any evidence on a single violation in the election process."

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Thursday's protests by Ahmadinejad's opponents were in open defiance of Supreme Leader Ayatollah Ali Khamenei, who has urged the nation to unite behind the Islamic state.

Mousavi's Web site said he might join the rally Thursday, but it was not immediately clear if he attended. The rally began late in the afternoon in downtown Tehran, according to a witness who spoke on condition of anonymity out of safety concerns.

The election crisis has ignited a full-on guerrilla cyberwar, with Twitterers and techies across the globe pitching in to help protesters in that country access censored websites like YouTube by tweeting links to Web proxy servers.

• Click here to see YouTube video that allegedly shows gunman killing protesters.

The U.S. State Department even reportedly weighed in, with an unnamed official telling Reuters Tuesday that it had asked Twitter not to "shut down its system in Iran."

Twitter itself, realizing how vital it had become, put off a scheduled early morning maintenance outage until 5 p.m. EDT Tuesday (1:30 a.m. Wednesday in Tehran) so that Iranians could get in a full day of uninterrupted tweeting.

The Iranian government reportedly has also taken notice of Twitter's importance to protesters.

"Security forces are setting up twitter accounts to spread disinformation by posing as Iranian protesters," several Twitter postings alleged Tuesday.

"Several arrests today after tracking thru twiter proxys," others warned.

Like most Twitter postings, those reports were unconfirmed, but in an interview with FOXNews.com, a 27-year-old Iranian woman who asked to be indentified only by her first name, Samereh, also said Iran's leaders were cracking down on several forms of communication.

"The government is taking satellites down from peoples’ homes,” she said. “They’re everywhere.”

• Click here to read more on this and other exclusives interviews from Iran.

Still, Iranians are finding ways to continue to use the Web to get their message out.

Samereh's interview was conducted via Yahoo chat.

Many Twitterers were changing their "location" setting to Tehran and their "time" to +0330 GMT in order to confuse Iranian Web censors seeking to track protesters or squelch in-country postings.

Others warned fellow Twitteres to "GO BACK & DELETE ALL PAST TWEETS that may have Iran usernames."

And others took a more aggressive tone, as "hacktivists" talked of taking down Iranian goverment Web sites, and at least one American blogger posted instructions on how to do so.

Tuesday, Web sites belonging to Ahmadinejad and Khamenei were unreachable.

The Associated Press contributed to this report.