Shortly after Neda Agha-Soltan bled her life out on the Tehran pavement, the man whose 40-second video of her death has ricocheted around the world made a somber calculation in what has become the cat-and-mouse game of evading Iran’s censors. He knew that the government had been blocking Web sites like YouTube and Facebook. Trying to send the video there could have exposed him and his family.

Instead, he e-mailed the two-megabyte video to a nearby friend, who quickly forwarded it to the Voice of America, the newspaper The Guardian in London and five online friends in Europe, with a message that read, “Please let the world know.” It was one of those friends, an Iranian expatriate in the Netherlands, who posted it on Facebook, weeping as he did so, he recalled.

Copies of the video, as well as a shorter one shot by another witness, spread almost instantly to YouTube and were televised within hours by CNN. Despite a prolonged effort by Iran’s government to keep a media lid on the violent events unfolding on the streets, Ms. Agha-Soltan was transformed on the Web from a nameless victim into an icon of the Iranian protest movement.

At one time, authoritarian regimes could draw a shroud around the events in their countries by simply snipping the long-distance phone lines and restricting a few foreigners. But this is the new arena of censorship in the 21st century, a world where cellphone cameras, Twitter accounts and all the trappings of the World Wide Web have changed the ancient calculus of how much power governments actually have to sequester their nations from the eyes of the world and make it difficult for their own people to gather, dissent and rebel.