About two million Iranians have access to the internet
Iran has tightened controls on the internet, ordering thousands of political and pornographic websites to be blocked.
The Iranian press said a list of 15,000 sites had been drawn up by the government and sent to internet service providers.
Ministers were quoted as saying that they wanted to "block access to immoral sites as well as political sites which rudely make fun of religious and political figures in the country."
The web has become an important outlet as an alternative method of communication in Iran, which maintains a close eye on the media.
Over the past three years, Iran's conservative judiciary has banned about 80 newspapers and magazines.
In response, several pro-reform publications had turned to the internet to get around strict press laws.
Banned newspapers have turned to the web
Estimates suggest there are about two million Iranians with access to the internet.
The hardliners who control Iran's state TV and radio and many newspapers accuse the new sites of spreading "lies" and exceeding socially accepted norms.
According to reports in the Iranian media, service providers could face court action unless they block access to 15,000 sites deemed immoral.
Several of the banned sites have close links to reformists such as the Iranian political bulletin Emrooz website.
Access to sites of radio stations that broadcast in Farsi are also reported to have been blocked.
The new restrictions on the net reflect growing concerns in Iran about the web.
Last month the authorities detained Sina Motallebi , a journalist behind a prominent web log, www.rooznegar.com.
Reporters Without Borders has issued a statement deploring Mr Motallebi's detention and other attacks on journalists.