Tabatabaei said the block was "a swift reaction" to a Mousavi campaign rally on Saturday that had been attended by young Iranians waving green banners and scarves - the symbolic color of the Mousavi campaign.

Youth vote

Social networking sites and blogs have become important campaign tools for Mousavi as he aims to mobilise Iran's youth vote for the June 12 election.

"Facebook is one
of the only independent sources that the Iranian youth
could use to communicate"

Mohammed Ali Abtahi,
former vice-president

There are a number of pages on Facebook dedicated to Mousavi, one of which has more than 6,000 supporters.

Mousavi himself has posted a comment on the site saying: "Ahmadinejad's government has dishonoured Iranians across the world".

"Facebook is one of the only independent sources that the Iranian youth could use to communicate," Mohammed Ali Abtahi, a former vice-president and now adviser to Mahdi Karroubi, another pro-reform candidate.

He said that the blocking of Facebook - and possibly other websites popular with reformists - would leave Iranians "forced to rely on government sources".

Those supporting Mousavi have said that they will use email to disseminate news of the banning.

'Sharing and expression'

Facebook, based in the US, said that they had received reports that their site had been barred and that they were investigating the issue.

"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," Elizabeth Linder, a spokeswoman for the website, said.

"It is always a shame when a country's cultural and political concerns lead to limits being placed on the opportunity for sharing and expression that the internet provides."

Mousavi is a former prime minster and one of three candidates running against incumbent Mahmoud Ahmadinejad. Mohsen Rezaie, a former head of the Revolutionary Guard is the fourth candidate standing.

Ahmadinejad is expected to face a tough challenge in being re-elected, with a poor economic record at home and the international tensions created by his remarks about other countries.