In a filing in the U.S. District Court in Buffalo, New York, Thursday, Zuckerberg said he has declared under oath that he neither signed the alleged contract with Paul Ceglia, nor wrote or received any related emails.
He asked a federal judge to order Ceglia to immediately turn over the alleged original contract and emails, and for permission to inspect Ceglia's computers.
``Zuckerberg and Ceglia never discussed Facebook and they never signed a contract concerning Facebook,'' the filing said. ''The contract is a cut-and-paste job, the emails are complete fabrications, and this entire lawsuit is a fraud.''
Christopher ``Kip'' Hall, a partner at DLA Piper representing Ceglia, did not immediately return requests for comment.
Thursday's filing escalates the stakes as Ceglia, a wood pellet salesman from Wellsville, New York, tries to show he contracted in 2003 for 50 percent of Zuckerberg's interest in what became Facebook, which is privately held.
Forbes magazine in March estimated Zuckerberg's net worth at $13.5 billion. Analysts have said Facebook could be worth $70 billion should it go public, perhaps in 2012.
In Thursday's filing, Zuckerberg acknowledged signing an agreement with Ceglia, but that it concerned work Zuckerberg did for StreetFax.com, a website that posted photographs of traffic intersections for use in the insurance industry.
Zuckerberg hopes to use forensic testing on Ceglia's documentation. ``When this testing confirms that all of these documents are forgeries, this lawsuit will end,'' he said.
Ceglia sued last July, saying he made a contract with Zuckerberg for an 84 percent Facebook stake.
After he changed law firms, Ceglia filed a complaint that discussed the alleged contract and emails from 2003 and 2004 when Zuckerberg was a student at Harvard University.
In a Feb. 2, 2004 email discussed in the complaint, Zuckerberg was said to have resisted a clause that could have given Ceglia a stake greater than 80 percent, and instead suggested ``that we officially return to 50/50 ownership.''
Zuckerberg said forensic examiners reviewed his Harvard email account and found none of the alleged emails.
He also called Ceglia allegedly forgetting about the contract for seven years ``incredible on its face,'' and recounted his prior run-ins with law enforcement.
This included an instance in 2009 when New York Attorney General Andrew Cuomo accused Ceglia of fraud and shut his business.
Separately, Cameron and Tyler Winklevoss are appealing to the U.S. Supreme Court a ruling that upheld their $65 million settlement with Facebook, which they accused of stealing their idea for the website. The twin brothers have said they were shortchanged in the 2008 accord.