At least 100,000 demonstrators marched silently through the streets of Tehran tonight in a direct challenge to the authority of Iran's clerical regime.

Witnesses said that the protesters, both young and old and many accompanied by children, marched through central Tehran Haft-e Tir square towards Vali Asr square in the heart of the city.

The rally was called by the defeated presidential candidate Mir Hossein Mousavi to protest against the "shameful fraud" that saw Mahmoud Ahmadinejad re-elected by a landslide in last Friday's election.

Mr Mousavi's appeal to supporters, issued via his website, flew in the face of a declaration 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".

But the regime is struggling to keep a lid on the protests. In an extraordinary display today, five players in Iran's World Cup qualifier against South Korea in Seoul took to the pitch with green armbands in apparent support of Mir Hossein Mousavi, although they removed the armbands for the second half of the match.

The same armbands were to be seen everywhere in the Tehran demonstration, along with placards accusing Mr Ahmadinejad of having "stolen" people's votes. Mr Mousavi was not attending the march, which has been branded illegal by the authorities.

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".

He also called a day of mourning for tomorrow,asking people to come together in mosques or in peaceful demonstrations in solidarity with those killed on Monday.

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