"This is one of the reasons we were certain that Moussavi would win, " he said.
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The rally was a repeat of a march that Moussavi supporters staged Wednesday, before the election.
CNN: Moussavi vows to 'pay any cost' to fight Iran election results
Ahmadinejad called Moussavi "corrupt" and accused his wife, a college professor, of skipping university entrance exams.
For the past 10 years, Moussavi, a painter, has been head of the state-owned Art Center.
Moussavi has called on Iranians to "exercise self-control, " while still supporting their right to demonstrate.
Khatami has openly sided with the challenger, Mir Hossein Moussavi, as has the reformist Grand Ayatollah Montazeri.
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Initially, many reformists were skeptical about Moussavi's reform credentials and feared that he was too conservative for their taste.
That poll found Ahmadinejad leading the four-candidate field with 34 percent support, while Moussavi trailed with 14 percent.
Also, the published results showed that Ahmadinejad won even in his opponents' strongholds, including Moussavi's hometown of ethnic Azeri Turks.
And even though Moussavi continues to defy the government, some say he's not a practical choice for any long-term movement.
Two hours into the rally, the silence was broken when Moussavi himself arrived, but few in the crowd could hear him.
Moussavi is a member of Iran's Expediency Council, which mediates between the parliament and the non-elected Guardian Council led by Ayatollah Khamenei.
Moussavi had rejected the offer of a partial recount and refused to appoint a representative to the committee, according to Press TV.
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Moussavi's wife campaigned for him, an usual step in politics in Iran.
The key to what happens next may be what role Moussavi assumes.
"Mr. Moussavi may face no choice other than seeking refuge, " Alfoneh said.
Ahmadinejad was declared the winner of the election by a margin of 2-to-1, surprising many experts who had expected Moussavi to win.
CNN: Moussavi vows to 'pay any cost' to fight Iran election results
It does not predict the final vote, nor does it measure a possible surge for Moussavi, which many believe occurred in the final weeks.
Moussavi, along with candidate Mehdi Karrubi, questioned the legitimacy of the June 12 vote count and demanded that the Guardian Council annul the results.
Moussavi may have won in the capital, "but he lost in the provinces, and that's something they have to come to terms with, " Afrasiabi said.
CNN: Moussavi vows to 'pay any cost' to fight Iran election results
That should have given Moussavi a comfortable lead over the incumbent.
According to many independent media reports from all over Iran, women and young voters turned out in record numbers, especially in cities where Moussavi is very popular.
Moussavi spoke through a bullhorn from atop a building to the throngs of marchers who snaked through the city, in what was described as a peaceful protest.
Moussavi told supporters the party will be focused on upholding "the remaining principles of the constitution, " according to Etemad-e Melli, a newspaper aligned with fellow opposition candidate Mehdi Karrubi.
Even so, Moussavi's supporters and key figures, including senior politicians and clerics, fueled rifts within Iran's Islamic leadership that were "evident for the whole world to see, " Dabashi said.
Moussavi and the other two defeated candidates, Mehdi Karrubi and Mohsen Rezaie, have reportedly been invited to the Guardian Council on Tuesday to discuss any concerns over the election results.
CNN: Moussavi vows to 'pay any cost' to fight Iran election results
Largely unknown to the rest of the world -- and semi-forgotten in Iran -- when he announced his bid for president in March, Moussavi campaigned as a foil to Ahmadinejad.
Former President Ali Akbar Hashemi Rafsanjani, chairman of Iran's Assembly of Experts and a supporter of opposition leader Mir Hussein Moussavi, was quoted Saturday by the semi-official Iranian Labor News Agency.
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Furthermore, Moussavi went into the election with massive support from the country's youth, who were unhappy with the faltering economy and an unemployment rate that tops 30 percent by some accounts.
Of Iran's population of 70 million, almost 60 percent are younger than 28 -- too young to have lived through the 1979 revolution, too young to remember Moussavi's time in power.
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