They say their access to Google services has been blocked and that they haven’t been able to access other sites like Facebook even with the usual antifiltering software.
The reason for the disruption is not clear. It could be an attempt by government authorities to prevent a planned silent protest that has been called by the opposition Green Movement for February 14.
The Green Path of Hope coordination council issued the call to mark the anniversary of a protest last year, which attracted tens of thousands of people and led to the house arrest of opposition leaders Mir Hossein Musavi and Mehdi Karrubi.
Iran has disrupted the Internet before ahead of planned opposition demonstrations.
There is also speculation that the blackout is related to the government's plan to launch a national Internet, which it reportedly has been testing for months.
Iranian officials have said that the national network will be launched soon, amid concerns that it will cut off Iranians from the World Wide Web.
Whatever the reason for the increased censorship, it has upset many.
An Iranian man who left a message on RFE/RL’s voice mail system described the disruption he has experienced:
Another man, Amin, claimed via e-mail that he hasn’t been able to access Facebook or his Yahoo mail even with "the strongest antifiltering tools."
"This demonstrates the degree of freedom we enjoy here; we can’t even check our e-mail," he said.
Another man called to ask for help in bypassing the tightened controls:
The Internet disruption has also been acknowledged by some websites inside Iran, including "Tabnak," which has called on authorities to tell citizens why they can't access Gmail and other Internet-based e-mail programs.
Iranian officials have had no comment on the reports.
-- Golnaz Esfandiari