'According to certain Internet surfers, the site was banned because supporters of Mir Hossein Mousavi were using Facebook to better disseminate the candidate's positions, ' it reported quoting Iranian news agency ILNA (Iranian Labour News Agency).

Mir Hossein Mousavi, who was Iran's prime minister when the post was abolished in 1989, is seen as one of the leading challengers to incumbent President Mahmoud Ahmadinejad in the June 12 elections, BBC reported Sunday.

Mousavi's page on Facebook has more than 5,000 supporters. People trying to visit the site in Iran received a message in Farsi saying: 'Access to this site is not possible.'

Facebook has expressed its disappointment over the ban saying that its site was blocked 'at a time when voters are turning to the Internet as a source of information about election candidates and their positions'.