More than 24 hours of video are uploaded to YouTube every minute
YouTube has accused media conglomerate Viacom of secretly uploading content to the video-sharing site whilst publicly complaining about its presence.
YouTube said it deliberately "roughed up" any uploaded videos to make them look stolen or leaked.
The accusation was made as a court prepares to rule in a $1bn suit brought by Viacom against Youtube for "massive intentional copyright infringement".
Viacom said it had identified 150,000 such infringements on the site.
"YouTube was intentionally built on infringement and there are countless internal YouTube communications demonstrating that YouTube's founders and its employees intended to profit from that infringement," Viacom said in a statement.
"By their own admission, the site contained 'truckloads' of infringing content."
But Zahavah Levine, YouTube's chief counsel, accused Viacom of covert operations to add copyright infringing content.
"For years, Viacom continuously and secretly uploaded its content to YouTube, even while publicly complaining about its presence there," she wrote in a blog post.
"It hired no fewer than 18 different marketing agencies to upload its content to the site. It deliberately 'roughed up' the videos to make them look stolen or leaked.
"It opened YouTube accounts using phony email addresses. It even sent employees to Kinko's to upload clips from computers that couldn't be traced to Viacom."
Viacom's 2007 lawsuit centres around a claim that YouTube consistently allowed unauthorised copies of popular TV shows and movies to be posted to its website and viewed tens of thousands of times.
The company said it had identified more thousands of abuses including clips from shows such as South Park, SpongeBob SquarePants and MTV Unplugged.
YouTube has always argued that it is covered by law through the Digital Millennium Copyright Act (DMCA), which states that publishers are not responsible for material posted by users.
YouTube boasts the biggest video sharing audience in the world
Companies are required by law to remove unauthorised clips from sites when they have been notified by the owner.
YouTube has insisted it has followed that rule and a month before the lawsuit, it took down more than 100,000 clips at Viacom's request.
But Viacom claims that YouTube founders flouted copyright and deliberately encouraged it in favour of increasing traffic to the site.
Documents revealed in court quoted a message sent on 19 June 2005 from YouTube co-founder Steve Chen to fellow co-founders Chad Hurley and Jawed Karim.
"Jawed (Karim), please stop putting stolen videos on the site. We're going to have a tough time defending the fact that we're not liable for the copyrighted material on the site because we didn't put it up when one of the co-founders is blatantly stealing content from other sites and trying to get everyone to see it," said the e-mail.
Another e-mail from Mr Chen to staff in the early days of the start-up's life said the company "should concentrate all our efforts in building up our numbers as aggressively as we can through whatever tactics, however evil".
The documents also reveal that Viacom considered buying YouTube just months before it launched the suit. Executives from Viacom thought it would be a "transformative acquisition".
They were eventually beaten by Google, which bought the site for $1.65bn.
Journalists and bloggers covering the story have said the whole affair smacks of a something you would see on TV.
"This looks like a reality show but the audience doesn't get a vote," Peter Kafka, senior editor of the news and technology blog AllThingsD.com told BBC News.
"The only person both sides have to win over is the judge. This is court filing of a three year-old case and the industry has moved on and realised this stuff is hard to monetise."
Eric Goldman, director of the High Tech Institute of Santa Clara University, told the San Jose Mercury News, that there will be no real winners in this legal battle.
"It's like a prize fight - they are both scoring points; they are both beating each other up. But instead of making money from the fight, they are paying to be in it. That's really dumb"
US District Judge Louis Stanton has given both parties until 30 April to file opposing arguments to each other's motions. All the arguments are expected to be completed by June.