Its use to organize and send pictures and messages to the outside world — in real-time as events unfolded — was a powerful example of how such tools can overcome government attempts at censorship.
"When I'm not connected to Twitter it means that I'm disconnected from the world because the state TV doesn't report many things!" wrote one Twitter user who identifies himself as "hamednz" and communicated with The Associated Press through email.
His profile says he lives in Rasht, a city to the north of Tehran near the Caspian Sea.
Like all the Twitter users in Iran who agreed to be interviewed for this story, hamednz did not want his identity revealed for fear of retribution from government authorities.
In Iran, as in many still-developing countries, Internet usage is mostly still a phenomenon of the affluent, the youth and city-dwellers — meaning Twitter and other networks are used mostly by the young and liberal — and may overemphasize their numbers while ignoring more-conservative political sentiments among the non-connected.
Supporters of reformist challenger Mir Hossein Mousavi are more likely to use Twitter and Facebook. Poorer, less-educated voters have flocked to President Mahmoud Ahmadinejad.
Twitter co-founder Biz Stone acknowledged the limited group of users in Iran, who don't necessarily represent the mainstream.
"Because Twitter is still a nascent service the sentiment is likely narrow," Stone said in an email Monday to The Associated Press.
"However, we noticed people creating accounts during the riots presumably because they heard Twitter was the most efficient way to discover and share what was happening in the moment," Stone wrote.
Twitter had planned to go down for 90 minutes Monday for maintenance, but rescheduled it citing "the role Twitter is currently playing as an important communication tool in Iran," according to a notice posted on its Web site. The maintenance will instead happen a day later, when it is 1:30 a.m. in Iran.
Iranians must outsmart government blocking to use Twitter, on which users post messages limited to 140 characters called "tweets."
Twitter and other social-networking sites remained blocked Monday in Iran. Users must go to other sites that post tweets for them and allow them to read tweets from others.
Facebook was used to organize people before the election, but it was also blocked after the vote. On Monday, the site was replaced with a message that said the page was blocked under a filtering policy.
"Since the election, we have heard reports that users in Iran are having difficulties accessing Facebook," said Barry Schnitt, Facebook spokesman. "This is disappointing, especially at a time when Iranians are turning to the Internet as a source of information about the recent election."
Cell phone service, which had been down in the capital since Saturday, was restored Sunday, but Iranians still could not send text messages from their mobile phones. Text messages are another way to tweet.
Judging by tweets since Friday's election — the censorship didn't even slow some of them down. Many then tweet to share their tactics.
"The fact that the government is not able to stop all of the information is really key," said Robin Gross, executive director of IP Justice, a San Francisco-based digital rights group. "They can only sort of censor in a patchwork way, and censorship by its nature has to be all or nothing."
Twitter has been used as an organizing tool to tell people about upcoming events such as this tweet Monday from a user identified as "alirezasha:" "today, 4pm a CALM protest with Karoubi and Mousavi/confirmed by Karoubi's campaign manager and VP."
Other users post pictures of protests or what appears to be government authorities chasing and beating protesters.
One Twitter user who identified himself only as Mohsen, speaking to the AP from Tehran, said while he's been using Twitter for about two years, he's intensified his "tweets" over the last few days.