Iran's defeated presidential candidate Mir Hossein Mousavi issued a direct challenge to the country’s clerical regime today, calling for a mass rally to protest against the "shameful fraud" that saw Mahmoud Ahmadinejad re-elected by a landslide.

Mr Mousavi's appeal to supporters, issued via his website, flew in the face of a declaration last night by Ayatollah Ali Khamenei, Iran's Supreme Leader, that the former prime minister should pursue his objectives through the electoral system and not on the streets.

It also came despite a demand from the powerful Revolutionary Guard that websites and bloggers should remove any materials that "create tension".

According to the official election results, Mr Mousavi scored 34 per cent of the vote in last Friday's election, against almost 63 per cent for Mr Ahmadinejad. His supporters say that in reality it was he who was the clear first round victor after a massive upsurge in support for his moderate message, particularly from young urban voters desperate for change.

In the biggest political protests since the Islamic Revolution of 1979, hundreds of thousands of people took to the streets of Tehran on Monday to protest against the alleged voter fraud. In clashes afterwards at a Basiji militia post, at least seven protesters were killed.

Yesterday, Mr Mousavi was forced to call off a mass protest in the heart of Tehran after Ayatollah Khamenei called a pre-emptive "unity rally" in the same square an hour earlier.

Today, the 67-year-old announced on his website: "We are after a peaceful rally to protest the unhealthy trend of the elections and realize our goal of annulling the election results." He also called for "a new presidential election that will not repeat the shameful fraud from the previous election".

The crackdown on the protests has been accompanied by a ban on journalists working for foreign media reporting from the streets of Tehran - officially they are only allowed to work on stories sanctioned by their Government minders.

The regime has also done its best to restrict access to the social networking sites and messaging systems through which news of planned protests has been disseminated – among them Facebook, Twitter and the video-sharing site YouTube.

They have not been able to prevent traditional word-of-mouth communication – usually through car windows at traffic jams in the capital.

Mr Mousavi condemned the government for blocking websites and trying to silence the voice of the opposition. The Revolutionary Guard, an elite military force answering only to Mr Khamenei, said through the state news service that its investigators have taken action against "deviant news sites" that encouraged public disturbance and street riots.

The Guards are a separate military with enormous domestic influence and control of Iran’s most important defence programmes and are considered a key source of power for Iran's rulers.

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