SAN FRANCISCO, May 2 — There is open revolt on the Web.

Sophisticated Internet users have banded together over the last two days to publish and widely distribute a secret code used by the technology and movie industries to prevent piracy of high-definition movies.

The broader distribution of the code may not pose a serious threat to the studios, because it requires some technical expertise and specialized software to use it to defeat the copy protection on Blu-ray and HD DVD discs. But its relentless spread has already become a lesson in mob power on the Internet and the futility of censorship in the digital world.

An online uproar came in response to a series of cease-and-desist letters from lawyers for a group of companies that use the copy protection system, demanding that the code be removed from several Web sites.

Rather than wiping out the code — a string of 32 digits and letters in a specialized counting system — the legal notices sparked its proliferation on Web sites, in chat rooms, inside cleverly doctored digital photographs and on user-submitted news sites like Digg.com.