Iranian authorities often block specific web sites and blogs considered critical of the Islamic regime, but the timing of the latest clampdown suggested it was done to hobble opponents of 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 for 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 Saturday in a Tehran sports stadium that included an appearance by former reformist president Mohammad Khatami and many young people waving green banners and scarves - the symbolic color 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 web sites popular with reformists - will leave Iranians "forced to rely on government sources" such as state-run media before the election.
Ahmadinejad is in a four-way race for re-election against the two pro-reform candidates and fellow conservative Mohsen Rezaei.
For its part Facebook described it as a "shame" when political concerns result in limits being placed 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 also are gaining in popularity.
Iranian officials did not comment on the reported block, while representatives for Facebook had no immediate response to queries either.
Get Action News on your website
