Three major international broadcasters have strongly condemned Iran for its "deliberate electronic interference" in their broadcasts.
The BBC, Deutsche Welle and Voice of America said the jamming began on Thursday as Iran marked the 31st anniversary of the Islamic Revolution.
They said Iran was broadcasting freely around the world while denying its own people programmes coming from outside.
Earlier, the US accused Iran of using a "near-total information blockade".
A state department spokesman said there were strong indications that the telephone network had been taken down, SMS messages blocked, and internet communication "throttled".
"Iran has attempted a near total information blockade," PJ Crowley said.
"It is clear that the Iranian government fears its own people."
White House spokesperson Robert Gibbs meanwhile said that the web giant, Google, and other internet service providers had been "unplugged" in Iran.
Reporters Without Borders says the blocking of Google's Gmail e-mail system takes the drive to control cyber-space to a new stage.
But the organisation claims that most Iranian internet users know how to sidestep censorship and access blocked websites.
Correspondents say a number of governments - notably China and Burma, as well as Iran - make strenuous efforts to block modern internet communications among their opponents.
'Accurate and impartial'
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