Other media companies also have major concerns about YouTube, but Viacom's was the first lawsuit filed by a major media owner. Several media companies have reached agreements to supply YouTube with clips, including CBS Corp., General Electric Co.'s NBC Universal and the British Broadcasting Corp., but many others remain reluctant to deal with the Web site because of copyright concerns. YouTube had been a quirky, fast-growing startup until the deep-pocketed Internet search behemoth Google Inc. bought the company last November for $1.76 billion.

But YouTube's soaring popularity, especially among younger people who are increasingly tuning out traditional media, has broadcasters frightened of losing viewers and advertising dollars. Last month, Viacom demanded that YouTube remove more than 100,000 unauthorized clips from its site, and since that time, the company has uncovered more than 50,000 additional unauthorized clips, Viacom spokesman Jeremy Zweig said.

A quick search of YouTube's site turned up numerous clips from Viacom programs including segments from Comedy Central's "The Daily Show with Jon Stewart" and Nickelodeon's "SpongeBob SquarePants" cartoon. In the lawsuit, filed in U.S. District Court in New York, Viacom says YouTube "harnessed technology to willfully infringe copyrights on a huge scale" and had "brazen disregard" of intellectual property laws.

Viacom is especially at risk because many of its shows are aimed at younger viewers who also are heavy Internet users. At the same time, Viacom is trying to find other, legal ways to distribute its shows digitally, such as by selling episodes of "The Daily Show" and "South Park" for $1.99 each through Apple Inc.'s iTunes service. Those shows can then be viewed on a computer or iPod.

Hitting the financial jackpot, it appears, may have created some headaches for YouTube, the wildly popular video-sharing Web site that has agreed to be bought by Google for $1.65 billion in stock. The site late last week began purging copyrighted material from Comedy Central, including clips from YouTube stalwarts like 'The Daily Show With Jon Stewart, The Colbert Report and South Park.

The action was "a result of third-party notification by Comedy Central," according to one such e-mail message sent to a YouTube user, Jeff Reifman, who broke the news on the Web site NewsCloud. A week earlier, nearly 30,000 clips of TV shows, movies and music videos were taken down after the Japanese Society for Rights of Authors, Composers and Publishers cited copyright infringement.

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In an interview with Wired magazine in September 2005, Mr. Stewart explained his view: "We get an opportunity to produce this stuff because they make enough money selling beer that it's worth their while to do it. I mean, we know that's the game. I'm not suggesting we're going to beam it out to the heavens, man, and whoever gets it, great. If they're not making their money, we ain't doing our show."