Black-clad demonstrators took to the streets of Iran's capital on Thursday, mourning the deaths of other protesters early in the week and condemning what they believe to be a fraudulent presidential election.
The crowds were estimated at much more than 100,000 people.
Amateur videos posted on the web early Thursday showed massive crowds gathering for the fourth straight day of protests, since President Mahmoud Ahmadinejad was re-elected in the controversial landslide election.
The protest by supporters of opposition leader Mir Hossein Mousavi came in direct defiance of Supreme Leader Ayatollah Ali Khamenei's calls for an end to the protests.
Mousavi himself attended the rally, and addressed the crowd, calling for calm and restraint.
The opposition leader had urged his supporters to wear black in mourning for those supporters killed in earlier violence, and for the alleged election fraud.
From Tehran, George McLeod, a freelance reporter for the Globe and Mail, described the protests as "remarkable."
"This protest was supposed to fail, according to authorities. But what I saw were masses and masses of people, many dressed in black, many carrying white flowers or pictures of the dead protesters who were killed by the Islamic militia," McLeod said.
On Monday, eight supporters of opposition leader Mir Hossein Mousavi were shot by pro-regime militia members.
McLeod went on to say "the protests seem to be getting bigger and bigger and the one today was massive. "
"It is becoming more of an anti-government movement than simply a protest against the elections," McLeod said.
"Most of the people I talked to are saying this is a great opportunity, a once in a life opportunity to create real structural change."
Foreign reporters have been restricted to reporting from their offices and are banned from publishing pictures of the unrest in Tehran. As a result, the stunning YouTube videos posted Thursday represented some of the only glimpses of what is happening inside the country.
Perceived dissidents have been rounded up by the Iranian government, and some websites and social networking sites were muzzled.
A joint letter from Mousavi and former reformist President Mohammad Khatami called on judiciary head Ayatollah Mahmoud Hashemi Shahroudi to stop police violence against the protesters and to help free imprisoned demonstrators.
Meanwhile, Iran's most influential body said it is investigating 646 complaints of voting irregularities.
