The Guardians Council, the electoral watchdog, said it has received a total of 646 complaints of irregularities in the conduct of the poll from Mr Mousavi and the other two defeated candidates and was investigating these complaints. It said it had invited the trio to set out their grievances on Saturday, with a decision on Sunday about any possible recount in the June 12 election which returned incumbent President Mahmoud Ahmadinejad to power.

Mr Mousavi himself joined the crowds, most of whom were dressed in black as a mark of mourning for demonstrators killed in clashes during the protests which had been banned by the authorities. State radio has reported seven deaths since the protests began.The protesters carried pictures of Mr Mousavi and placards bearing slogans such as ‘We have not had people killed to compromise and accept a doctored ballot box’.

Supreme leader Ayatollah Ali Khamenei said he would consider a partial recount after Mr Ahmadinejad’s defeated challengers lodged formal complaints of vote-rigging. But Mr Mousavi is insisting that the result of what he has described as a ‘fraud’ be annulled and a new vote called.