TEHRAN, Iran - The popular social networking site Facebook is being blocked in Iran in what reformists call an attempt to undermine their main candidate in next month's election.

Websites such as Facebook are a key campaign tool for Mir Hossein Mousavi's supporters, who seek to mobilize the youth vote in the June 12 election against Iranian President Mahmoud Ahmadinejad.

Mousavi's backers claim that authorities are trying to cripple their Internet efforts and worry that the block could expand to other sites.

Iranian officials did not comment on the reported block Sunday.

Ahmadinejad faces Mousavi and two other candidates in his bid for re-election.

Representatives for Facebook did not immediately respond to e-mails seeking comment.