The communication cuts came after President Mahmoud Ahmadinejad won a landslide re-election victory, sparking rioting in the streets by opposition supporters who claimed the result had been rigged.
The mobile phone network stopped working at 10:00 pm (1730 GMT), just before Ahmadinejad went on television to declare the election a "great victory" and even as baton-wielding police were clashing with protestors in the streets of Tehran, according to witnesses.
Iran has two national networks run by state-owned MCI (Telecommunication Company of Iran) and the private firm Irancell.
Several Iran-based users logging on via different Internet service providers, meanwhile, said they could reach neither Facebook nor YouTube — the two websites used effectively by young supporters of Ahmadinejad's moderate rival Mir Hossein Mousavi.
Mousavi complained bitterly on Saturday against "vote rigging" in the election, unleashing violent clashes between his supporters and anti-riot police.
Scores of users started posting pictures and videos of the protests on both sites shortly after they broke out in Tehran's streets.
Iranian authorities banned the popular social networking website Facebook on May 23 reportedly to prevent Mousavi supporters from using it for his presidential campaign prior to Friday's poll.
Access was restored after a few days.
About 60 percent of Iran's 70-million population is under 30 years old and the country, which applies strict monitoring of cyber material, has some 20 million web users.
Several pro-Mousavi news websites have also been blocked in the past two days including two popular ones, Aftab News and Shahab News, which are regarded as close to Iran's top arbitration body, the Expediency Council.
The Council is headed by influential former president and Mousavi-backer, Akbar Hashemi Rafsanjani, who was the subject of mudslinging in the presidential campaign after Ahmadinejad accused his sons of receiving financial privileges in the past.
