According to witnesses, many of the placards and slogans at the pro-government rally were in support of Khamenei rather than Ahmadinejad. They denounced Israel and the U.S., frequent rallying cries among the large crowds that attend the main Friday prayer sermons in Tehran and the annual celebrations commemorating the 1979 Islamic Revolution.
"This nation will protect and defend its revolution," Gholam-Ali Haddad-Adel, an Ahmadinejad supporter, told the crowd, which numbered more than 100,000, not nearly as dramatic as the massive unauthorized opposition demonstration that took place a day earlier in Freedom Square.
But pro-Mousavi demonstrators managed to overcome cellphone outages and blocked social networking websites to use word of mouth, e-mail, text messaging and pay phones to quickly spread the word and organize a huge silent march in north Tehran.
According to witnesses, the protest grew as the evening wore on to include at least tens of thousands, stretching from Vanak Square to Tajrish Square at the foothills of the Alborz Mountains. Tehran residents said they flowed into Vali Asr from all parts of the city to join in the mostly peaceful gathering.
They held up placards reading "Silence!" and snapped photos of one another with cellphone cameras, according to witnesses and footage broadcast on international satellite news channels. They spread the word for another rally today in Tehran's 7th Tir Square.
"This silence is a protest to the ruling establishment," one man said.
Many of the young men wore black shirts to honor five students reportedly killed in the dormitories of Tehran University by pro-government Basiji militiamen Sunday night, according to a witness.
Mousavi was nowhere to be seen. A source in contact with his confidants said he and his outspoken wife, Zahra Rahnavard, were being kept under close supervision in their Tehran home, with restricted access to the Internet and telephones. Many of his supporters say they regard him as a "temporary" leader to rally around.
"I am happy that Mousavi was not elected as president," said Ershad, 21, a student of industrial engineering, who was walking along Vali Asr Street south of the rally, which he said he had just attended. "If he were, he would have been just another Khatami," the reformist President Mohammad Khatami, who tried and failed to open Iran's political system beginning in the late 1990s.
"Now the gap within the system widens and the process of de-legitimization begins," he said, asking that his last name not be published. "Many are increasingly disillusioned about the system."
As night fell, neighborhoods erupted in a mix of voices, residents climbing to their rooftops and chanting "God is Great!" and "Death to the dictator," in what has become a nightly ritual of protest, growing louder and more explicitly political each day.
"If this fades, little by little, which is highly possible," said Akbar Hassani, who is in his 20s, "it will be like fire buried under ash, and some other time it will reappear."
Mostaghim is a special correspondent.