Cellphone service was shut down much of the day, and authorities are keeping an eye on websites.
A 21-year-old engineering student named Ali said he had never even heard of Twitter. But he learned about Wednesday's silent demonstration at Seventh of Tir Square by attending the previous day's march along Vali Asr Street. So he told friends and relatives by using pay phones, and spread the word to his friends at the dormitories.
"All the websites are shut down," he said, asking that his last name and his school not be published. "The phones never work. We find out through word of mouth."
On the street, a loose network of organizers appears to guide the demonstrators, cordoning off the marchers from traffic and urging them not to chant slogans or engage in provocative talk with the Basiji militiamen who sometimes stand glaring at the protesters.
The impromptu leaders, mostly students and women's rights activists, hide their faces with green bandannas or surgical masks to prevent security officials from identifying them.
Protesters have tailored their message to make sure no one makes blanket calls against the Islamic Republic. Such rhetoric would not only provoke the authorities, but alienate segments of a budding movement that includes a huge cross-section of the nation: emergency room physicians and pious, working-class women who cover all but their faces in black chadors; factory owners and factory workers; and a wide range of political groups whose agendas converge in opposition to Ahmadinejad.
Posters held aloft Wednesday urged demonstrators to stop their march at a certain point and call out praise for the prophet Muhammad. Then, witnesses said, protesters were instructed to remain silent for 10 minutes in honor of those killed so far in the unrest, disperse and go home.
Big rallies held Tuesday and Wednesday were largely silent, devoid of slogans altogether, except for the occasional salavats -- blessings for the prophet and his descendants -- which served to both refresh the crowd as it walked along in the late spring heat and make older, pious protesters feel welcome as they worked their prayer beads.
"We had one vote and we gave it to Mousavi," said one placard at Wednesday's rally. "We have one life and we'll give it up for freedom."
