Activist groups sued Viacom Inc. on Thursday, claiming the parent of Comedy Central improperly asked the video-sharing site YouTube to remove a parody of the cable network's ''The Colbert Report.''

Viacom responded by saying it had no records of ever making such a request.

Although the video in question contained clips taken from the television show, MoveOn.org Civic Action and Brave New Films LLC argued that their use was protected under ''fair use'' provisions of copyright law.

With Viacom identified by YouTube as the source of the removal request, they said Viacom should have known the use was legal and thus its complaint to YouTube to have the video blocked amounted to a ''misrepresentation'' that is subject to damages under the 1998 Digital Millennium Copyright Act.

The challenge, filed in U.S. District Court in San Francisco, came about a week after Viacom filed its own, $1 billion lawsuit against YouTube, claiming that the wildly popular Web site is rife with copyrighted video from Viacom shows, including ''The Colbert Report.''

Neither YouTube nor its parent, Google Inc., was named in the latest lawsuit, filed on the plaintiffs' behalf by the Electronic Frontier Foundation and Stanford Law School's Center for Internet and Society.

Viacom termed the lawsuit a waste of scarce judicial resources and said the plaintiffs should have checked first with the company.

In a letter to the plaintiffs' lawyers, Michael D. Fricklas, general counsel for Viacom, said the company had no record of sending YouTube a complaint, despite YouTube's identification of Viacom as the source. YouTube had no comment about the discrepancy.

After reviewing the clip, Fricklas said, ''I can inform you that Viacom has no problem with your client's continued use of it on its website or on YouTube.''

Corynne McSherry, a staff attorney with the EFF, said the organization had checked directly with YouTube staff to confirm Viacom's role and would investigate further with YouTube.

''We're happy they don't have a problem with our clip, ... but at this point it is still our understanding that they sent a takedown notice based on it,'' she said. ''As far as we're concerned we still have a lawsuit pending.''

Under the DMCA, YouTube and other service providers are generally immune from copyright lawsuits as long as they promptly respond to copyright complaints, known as takedown notices. According to the lawsuit, a takedown notice was sent to YouTube last week, and the video was blocked almost immediately.

Service providers are not required to investigate claims under the DMCA and in fact could lose their immunity if they take too long to respond. The law does give users the right to sue the issuer of the takedown request when it contains misrepresentations that an item is infringing. Such lawsuits are rare, though.

''People just shoot off a takedown notice without really giving a second thought to the material being taken down and whether it's really proper to be taken down,'' McSherry said. ''A lot of people cave in because they don't realize they can push back or they can't afford to push back.''

The lawsuit seeks unspecified legal costs and damages on grounds the plaintiffs' free-speech rights were harmed.

''With this lawsuit, we are making clear that corporations like Viacom must not be allowed to muzzle independent video creators and censor their free speech,'' said Eli Pariser, MoveOn's executive director.

The parody ''Stop the Falsiness,'' a play on host Stephen Colbert's use of the term ''truthiness,'' was jointly produced by MoveOn and Brave New Films, an activist production company that has made documentaries on the Iraq war, Wal-Mart and the Fox News Channel.