The Australian hacker is wanted in Sweden to answer sex-crimes allegations. American officials also are trying to build a criminal case against WikiLeaks, which has published a trove of leaked diplomatic cables and secret U.S. intelligence files on Iraq and Afghanistan.
Assange's lawyers are seeking to link the two issues, claiming the Swedish prosecution is politically motivated—an allegation Sweden strongly denies.
Assange's defense argument claims "there is a real risk that, if extradited to Sweden, the U.S. will seek his extradition and/or illegal rendition to the USA, where there will be a real risk of him being detained at Guantanamo Bay or elsewhere."
The document, prepared by Assange's lead lawyer, Geoffrey Robertson, adds that "there is a real risk that he could be made subject to the death penalty" if sent to the United States.
Under European law, suspects cannot be extradited to jurisdictions where they may face the death penalty.
It also is not clear what, if any, charges U.S. authorities could bring against Assange, and unclear how he could be classed as an "unlawful enemy combatant"—which could expose him to detention at Guantanamo Bay.
"Mr. Assange would not be sent to Guantanamo," said John Bellinger, a former legal adviser to the U.S. State Department. "He would be prosecuted in U.S. federal court. He would not be treated as an enemy combatant. Those are ridiculous concerns."
He added that "while Mr. Assange faces serious charges with a potential prison term, U.S. prosecutors certainly would not seek the death penalty."
British extradition specialist Karen Todner rejected the argument that Assange might be at greater risk of being sent to the United States from Sweden.
"I think he's more likely to face an extradition to the U.S. if he's in Britain," she said. Britain has a special fast-track extradition arrangement with the United States.
Assange, 39, was arrested arrest last month on rape and molestation accusations stemming from encounters with two women during a trip to Sweden last summer. He appeared briefly in a London court Tuesday ahead of a full extradition hearing Feb. 7 and 8.
Assange's lawyers say he should not be extradited because he has not been charged and is only wanted for questioning, and they plan to argue in court that the European arrest warrant seeking his detention was improperly issued.
The defense document also claims Assange has been the victim of "a pattern of illegal and or corrupt behavior" by the Swedish prosecuting authorities, including the release of his name to the public, "thus ensuring his vilification throughout the world."
As U.S. officials seek to build a case against WikiLeaks, American prosecutors have demanded details about the Twitter accounts of Assange, Pvt. Bradley Manning -- the Army intelligence analyst in custody who is suspected of supplying WikiLeaks with classified information -- and several WikiLeaks supporters.
Despite WikiLeaks' protests at the U.S. demand, Assange's lawyers complain that they have not been given access to text messages and tweets by Assange's accusers which the defense claims undermine the two women's case. They say text messages exchanged by the claimants "speak of revenge and of the opportunity to make lots of money."
Swedish Prosecutor Marianne Ny declined to comment on the case Tuesday.
WikiLeaks sparked an international uproar when it began publishing hundreds of classified U.S. diplomatic cables late last year, revelations that caused weeks worth of embarrassing news stories for the U.S. and its allies. But the flow of leaks, published in The New York Times, The Guardian, Der Spiegel, Le Monde and El Pais, has slowed recently amid a barrage of online attacks, financial difficulties and the Swedish case against Assange.
Assange, who is on bail and living under curfew at a supporter's rural mansion in eastern England, said that would soon change, as new media outlets are being made party to the leaks.
"We are stepping up our publishing for matters related to Cablegate and other materials," Assange said outside the high-security Belmarsh Magistrates' Court. "Those will shortly be occurring through our newspaper partners around the world—big and small newspapers and some human rights organizations."