They captured photos and videos of the man. On Wednesday afternoon, an anonymous individual uploaded the disturbing video to YouTube, where it was viewed by thousands and shared by bloggers.

“This is absolutely despicable,” wrote one of those commenting on the video, urging the protesters to stay active. “The rest of the world is watching and cheering you on.”

Via the Internet, the world has received unprecedented looks at the continuing unrest in Iran. As many foreign journalists are forced to leave Tehran and others are essentially confined to their hotel rooms, news organisations are looking more and more to the Iranians themselves to provide the news.

Dozens of videos of the protests by opponents of Iranian President Mahmoud Ahmadinejad have appeared on YouTube and other sites, provided by Iranians eager to circumvent the shroud of censorship their Government was trying to place over the unfolding events.

On Wednesday, amateur videos of an opposition rally were one of the primary sources of television pictures from Tehran. Another video showed a protest inside a Tehran train station.

YouTube said it had relaxed its usual restrictions on violent videos to allow the images from Iran to reach the rest of the world.

“In general, we do not allow graphic or gratuitous violence on YouTube,” the company said in a statement. “However, we make exceptions for videos that have educational, documentary, or scientific value. The limitations being placed on mainstream media reporting from within Iran make it even more important that citizens in Iran be able to use YouTube to capture their experiences for the world to see.”

But Iran continued to try to restrict web communications. On Wednesday, the Revolutionary Guard warned that protesters trying to stoke tensions using Internet sites like Twitter would be subject to retribution.