Protesters march during a silent demonstration against the results of the Iranian presidential election in central Tehran June 18, 2009. [Agencies]

Protesters have focused on the results of the balloting rather than challenging the Islamic system of government. But a shift in anger toward Iran's non-elected theocracy could result in a showdown over the foundation of Iran's system of rule.

"I don't think everyone wants to end the Islamic Republic because many people in Iran are very religious. So I think this current movement should keep Islam in it to maintain support. Unity is important," said a 29-year-old engineering graduate.

He, like the other witnesses, spoke on condition of anonymity because they feared reprisal.

The demonstrators marched silently until they reached the central square a witness said. Protesters warned the government: "We will not get exhausted and we will come every day."

Television footage showed protesters making V-for-victory gestures and holding pictures of Mousavi and signs that say "Where's our Vote?"

The groundswell of support appears to have taken Iran's leaders - and even Mousavi supporters - by surprise.

This week's rallies openly defied orders from Khamenei, who has urged the people to pursue their allegations of election fraud within the limits of the cleric-led system.

Thursday's march was similar to one on Monday, when hundreds of thousands turned out in a huge procession that recalled the scale of protests during the 1979 Islamic Revolution which ended the monarchy. Seven demonstrators were shot and killed that day by militia in the first confirmed deaths during the unrest.

The crowds in Tehran and elsewhere have been able to organize despite a government clampdown on the Internet and cell phones. The government has blocked certain websites, such as BBC Farsi, Facebook, Twitter and several pro-Mousavi sites that are vital conduits for Iranians to tell the world about protests and violence. Other sites are slow to connect.

Text messaging, which is a primary source of spreading information in Tehran, has not been working since last week, and cell phone service in Tehran is frequently down.