The Iranian government is mounting a campaign to disrupt independent media organisations and websites that air doubts about the validity of the re-election of Mahmoud Ahmadinejad as the nation's president, according to various sources.

Supporters of Mir Hussein Moussavi, the presidential challenger whom President Ahmedinejad claims to have defeated with 63.4 per cent of the vote, have emulated the internet campaign techniques used by Barack Obama to appeal to the young generation of Iranians who make up the majority of his support base.

But reports from Tehran say that social networking websites such as Facebook and Twitter, the micro-blogging website, were taken down after Mr Ahmadinejad claimed victory. SMS text messaging, a preferred medium of communication for young Iranians, has also been disabled. This is widely suspected to be the result of government interference, but could equally be caused by the poor quality of the network and the heavy demand it is experiencing.

Reporters Without Borders, the media organisation that campaigns for a free press around the world, said in a statement today: "The blocking of access to foreign news media has been stepped up. In addition to the blocking of the BBC’s website, the Farsi-language satellite broadcasts of the Voice of America and BBC — which are very popular in Iran — have been partially jammed.

"The Internet is now very slow, like the mobile phone network. YouTube and Facebook are hard to access and pro-reform sites . . . are completely inaccessible."

Facebook has not been able to get to the bottom of what is affecting its services. A spokesman for the company said: “We have heard reports that users in Iran are having difficulties accessing Facebook. This is disappointing, especially at a time when citizens are turning to the Internet as a source of information about the recent election.

“It is always a shame when a countries’ cultural and political concerns lead to limits being placed on the opportunity for sharing and expression that the Internet provides."

The BBC confirmed that its Persian service, which is based in London, was suffering interference in the region. The corporation beams the channel into the area via three separate satellites, two of which appeared to have had jamming technology directed at them over the weekend.

Sources at the corporation said it seemed that the Iranians had tested their jamming technology, which is highly expensive and usually only acquired at a state level, on Friday against one satellite, with mixed results. They had then targeted signals coming from the Hotbird 8 satellite, more successfully.

Peter Horrocks, director of the BBC World Service, said:"It seems to be part of a pattern of behaviour by the Iranian authorities to limit the reporting of the aftermath of the disputed election.

“In Tehran John Simpson and his cameraman were briefly arrested after they had filmed the material for a piece. And at least one news agency in Tehran has come under pressure not to distribute internationally any pictures it might have of demonstrations on the streets in Iran.”

Al Arabiya, the Dubai-based Arabic news channel, said that the Iranian authorities had forced it to close its Tehran bureau "for a week", with no explanation.

Nabil Khatib, its executive news manager, said: “Al Arabiya is worried about being banned from the chance to cover an important country like Iran during an important event like the elections and afterwards without explaining the reason behind that decision.

“Our correspondent just got a call after we announced the news inviting him for a meeting at the ministry tomorrow to discuss the situation. Until then he’s banned from doing any work."

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