Iran has blocked access to Facebook, prompting government critics to condemn the move as an attempt to muzzle the opposition before next month's presidential election.
Blogs and websites such as Facebook have become an important campaign tool for the leading reformist candidate, Mir Hossein Mousavi, to mobilise Iran's critical youth vote before the June 12 vote.
Iranian authorities often block specific websites and blogs considered critical of the Islamic regime, but the timing of the latest clampdown suggested it was intended to undermine opponents of the hardline president, Mahmoud Ahmadinejad.
"Every single media outlet that is seen as competition for Ahmadinejad is at risk of being closed," said Shahab Tabatabaei, a top aide of Mousavi. "Placing limits on the competition is the top priority of the government."
Tabatabaei said the Facebook block was "a swift reaction" to a major pro-Mousavi rally yesterday in a Tehran sports stadium which included an appearance by former reformist president Mohammad Khatami and many young people waving green banners and scarves – the colour of the Mousavi campaign.
"Facebook is one of the only independent sources that the Iranian youth could use to communicate," said Mohammed Ali Abtahi, a former vice-president and now adviser to another pro-reform candidate, Mahdi Karroubi, a former parliament speaker.
Abtahi said the loss of Facebook – and possibly other websites popular with reformists – would leave Iranians "forced to rely on government sources" such as state-run media before the election.
Ahmadinejad is in a four-way race against the two pro-reform candidates and fellow conservative Mohsen Rezaei.
Facebook said it was a "shame" when political concerns resulted in limits on opportunities for online expression.
"We are disappointed to learn of reports that users in Iran may not have access to Facebook, especially at a time when voters are turning to the internet as a source of information about election candidates and their positions," the company said in a statement.
The internet and other technology have increasingly become part of Iranian political movements in the past decade.
During the last presidential race, in 2005, information about rallies and campaign updates were sent by text message. In recent years, political blogs by Iranians in the country and abroad have grown sharply. Newcomers such as Twitter are also gaining in popularity.