WASHINGTON — When the powerful world of old media mobilized to win passage of an online antipiracy bill, it marshaled the reliable giants of K Street — the United States Chamber of Commerce, the Recording Industry Association of America and, of course, the motion picture lobby, with its new chairman, former Senator Christopher J. Dodd, the Connecticut Democrat and an insider’s insider.

Yet on Wednesday this formidable old guard was forced to make way for the new as Web powerhouses backed by Internet activists rallied opposition to the legislation through Internet blackouts and cascading criticism, sending an unmistakable message to lawmakers grappling with new media issues: Don’t mess with the Internet.

As a result, the legislative battle over two once-obscure bills to combat the piracy of American movies, music, books and writing on the World Wide Web may prove to be a turning point for the way business is done in Washington. It represented a moment when the new economy rose up against the old.

“I think it is an important moment in the Capitol,” said Representative Zoe Lofgren, Democrat of California and an important opponent of the legislation. “Too often, legislation is about competing business interests. This is way beyond that. This is individual citizens rising up.”