A one billion dollar lawsuit against YouTube threatens internet freedom, according to its owner Google.
Google's claim follows Viacom's move to sue the video sharing service for its inability to keep copyrighted material off its site.
Viacom says it has identified 150,000 unauthorised clips on YouTube.
In court documents Google's lawyers say the action "threatens the way hundreds of millions of people legitimately exchange information" over the web.
The search giant's legal team also maintained that YouTube had been faithful to the requirements of the 1998 Digital Millennium Copyright Act and that they responded properly to claims of infringement.
In papers submitted to a Manhattan court, Google said it and YouTube "goes far beyond its legal obligations in assisting content owners to protect their works".
Viacom disagreed that either firm had lived up to that standard and said that they had done "little or nothing" to stop infringement.
In a rewritten lawsuit filed last month, Viacom claimed YouTube consistently allowed unauthorised copies of popular television programming and movies to be posted on its website and viewed tens of thousands of times.
It said it had identified more than 150,000 such abuses which included clips from shows such as South Park, SpongeBob SquarePants and MTV Unplugged.
Interview with one of the founders of YouTube, Chad Hurley, first broadcast on 21 May 2007
The company says the infringement also included the documentary An Inconvenient Truth which had been viewed "an astounding 1.5 billion times".
Viacom, which is asking for damages for the unauthorised viewing of its programming, said its tally represented only a fraction of the content on YouTube that violates its copyrights.
"The availability on the YouTube site of a vast library of the copyrighted works of plaintiffs and others is the cornerstone of defendants' business plan," Viacom said.
Viacom originally started legal action last year and filed an amended version last month. Earlier this month Viacom chairman Sumner Redstone told Dow Jones: "When we filed this lawsuit, we not only served our own interests, we served the interests of everyone who owns copyrights they want protected."
He added: "We cannot tolerate any form of piracy by anyone, including YouTube...they cannot get away with stealing our products."
For its part, Google said the only way the legal action would be resolved was in court.
Google's vice president of content partnerships David Eun has said: "We're going all the way to the Supreme Court. We've been very clear about it."
After the legal action was first started, YouTube launched an anti-piracy tool that checks uploaded videos against the original content in an effort to flag piracy.
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