The BBC has demanded an explanation from the Iranian authorities after a block was put on the corporation's Persian-language website.

Since 18 January, users inside Iran are presented with an 'access denied' without any official explanation of hwy.

BBC Persian.com records more traffic than any other non-English language BBC site - around 30 million page impressions each month. Roughly half this traffic is from within Iran itself.

The BBC reports that it has been contacted by Iranian web users asking how to use 'filter-busting' sites that enable access to blocked websites.

Iran has a highly active online pro-democracy movement and a number of bloggers and journalists have recently been imprisoned.

Press freedom group Reporters Sans Frontières (RSF) yesterday reported that seven regional journalists were arrested on 29 January and may face the death penalty after mistakenly insulting the Ayatollah.

Last week RSF also reported that blogger and journalist Arash Sigarchi was sentenced to a three-year prison sentence for 'propaganda against the regime'.

Mr Sigarchi contributed to RSF's handbook for bloggers and cyber-dissidents in which he said online journalism would eventually lead to a plurality of voices and free expression in Iran.