Geoffrey Robertson told an extradition hearing that Assange would not get a fair trial because of his notoriety and because Swedish rape cases are customarily held without public or media present, to protect the alleged victims.
Closed-door hearings would be "a flagrant denial of justice ... blatantly unfair, not only by British standards but by European standards and indeed by international standards," Robertson said.
He said the risk was especially high, "given the amount of vilification throughout the world Mr. Assange has faced."
The British lawyer representing Sweden, Clare Montgomery, countered that Swedish trials were based on the principle that everyone deserves "a fair and public hearing." She said that in cases where evidence is heard in private it will often be published after the trial and recited in the judgment.
Assange is accused of sexual misconduct by two women he met during a visit to Stockholm last year. He denies wrongdoing.
The 39-year-old Australian appeared composed and occasionally took notes as he sat in the dock at London's Belmarsh Magistrates' Court on the first day of a two-day hearing.
His lawyers' wide-ranging arguments against extradition range from criticism of Swedish prosecutor Marianne Ny to claims that Assange could eventually be extradited from Sweden to the United States, and even sent to Guantanamo Bay.
Assange's lawyers also say he cannot be extradited because he has not been charged with a crime in Sweden and is only wanted for questioning - and that the allegation is not rape as understood under European and English law.
Robertson said Assange faces an allegation known in Swedish as "minor rape."
"That is a contradiction in terms," he said. "Rape is not a minor offense." He said Assange had had consensual sex with his two accusers.
Montgomery contested this, saying: "The Swedish offense of rape contains the core element of rape ... the deliberate violation of a woman's sexual integrity through penetration."
WikiLeaks sparked an international uproar last year when it published a secret helicopter video showing a U.S. attack that killed two Reuters journalists in Baghdad. It went on to release hundreds of thousands of secret U.S. military files on the wars in Iraq and Afghanistan, and it later began publishing classified U.S. diplomatic cables whose revelations angered and embarrassed the U.S. and its allies.
American officials are trying to build a criminal case against WikiLeaks. Assange's lawyers claim the Swedish prosecution is linked to the leaks and politically motivated.
Written defense arguments released by Assange's legal team claim "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."
Many legal experts say the Guantanamo claims are fanciful, and Sweden strongly denies coming under American pressure.
Nils Rekke, head of the legal department at the Swedish prosecutor's office in Stockholm, has said Assange would be protected from transfer to the U.S. by strict European rules, which would require approval from both Sweden and Britain.
Montgomery said it was ridiculous to suggest "that Sweden is somehow to be regarded as a state that provides no protections against this sort of threat and violation."
Assange was arrested in London in December after Sweden issued a warrant on rape and molestation accusations.
Released on bail on condition he live - under curfew and electronically tagged - at a supporter's country mansion in eastern England, Assange has managed to conduct multiple media interviews, sign a reported $1.5 million deal for a memoir, and pose for a magazine Christmas photo shoot dressed as Santa Claus.
The extradition case revolves around the contested events of Assange's trip to Sweden in August. Two Swedish women say they met Assange when he visited the country and separately had sex with him, initially by consent.
In police documents leaked on the Internet, one of the women told officers she woke up as Assange was having sex with her, but let him continue even though she knew he wasn't wearing a condom. Having sex with a sleeping person can be considered rape in Sweden.
Assange is also accused of sexual molestation and unlawful coercion against the second woman. The leaked documents show she accuses him of deliberately damaging a condom during consensual sex, which he denies.
Robertson denied Assange had committed any sexual offenses under English law. He said all relationships, long or short, contain "moments of frustration, irritation and argument. This doesn't mean, in this country, that the police are entitled to sniff under the bedclothes."
Details of the case have been reported around the world, and the defense argues Assange's notoriety will make it hard for him to get a fair trial.
A retired Swedish judge, Britta Sundburg-Weitman, appeared as a defense witness and said public opinion in Sweden about Assange was "rather hostile. I think most people take it for granted that he has raped two women."
She also questioned the motives of Ny, the prosecutor, saying her behavior "looks malicious."
Outside court, Assange said that since August "a black box has been applied to my life. On the outside of that black box has been written the word rape."
"That box has now, thanks to an open court process, been opened," he said. "I hope in the next days you will see that the box is in fact empty."
The extradition hearing is due to end Tuesday, but Judge Howard Riddle could take several weeks to consider his ruling - which can be appealed by either side.
