Iran has blocked the popular social media site Facebook, a move critics charge that President Mahmoud Ahmadinejad made in the runup to elections in an effort to weaken the youth vote.

It's not unusual for authorities to block internet access to material critical of the Islamic regime, but the timing of the latest move is suspect, say critics.

The decision — three weeks before the election — forces Iranians to rely on state-run media and other government sources.

Young voters, who are more likely to use Facebook as a source of information, are being courted by the main reformist candidate, Mir Hossein Mousavi.

"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 two-time reformist president Mohammad Khatami and many young people waving green banners and scarves — the symbolic 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 parliamentary Speaker.

With files from The Associated Press