IranElection is the top search term on Twitter. It is being used by Iranians to co-ordinate protests and post photos and messages in the wake of the presidential election on Friday.
The opposition candidate Mir Hossein Mousavi is also posting updates on Twitter and using it to rally supporters.
One message on Mousavi1388 asks: "Confirmed by BBC Persian, please tell everyone to join them: Mousavi, Karoubi & Khatami will be at the protest. #IranElection."
When it appeared Twitter was due to shut down for a 90-minute maintenance break but the site decided to reschedule the work so the protests could continue.
Mousavi's Twitter feed also made a direct appear to Twitter: "@twitter Twitter is currently our ONLY way to communicate overnight news in Iran, PLEASE do not take it down. #IranElection."
For more details and examples of social media in action read Gordon's Republic blog post: Social media and the Iranian election