Lawyers for the US government on Wednesday criticised a British judge for blocking the extradition of WikiLeaks founder Julian Assange, questioning evidence about his mental health and accusing an expert witness of misleading the court.

Judge Vanessa Baraitser in January refused to grant a request by Washington for Assange to face trial in the United States on spying charges, ruling he was at serious risk of suicide.

But lawyer Clair Dobbin, representing the US government which is appealing the decision, said the judge "didn't appreciate the weight" of expert evidence that concluded Assange was not a suicide risk.

Dozens of Julian Assange's supporters gathered outside the London court, some waving placards
Dozens of Julian Assange's supporters gathered outside the London court, some waving placards AFP / JUSTIN TALLIS

Instead, the judge relied on evidence presented by Assange's psychiatric expert Michael Kopelman, Dobbin told the High Court in London.

She said that Kopelman had admitted to misleading the court by "concealing" that his client had fathered children while holed up in the Ecuadorian embassy in London.

"She really needed to interrogate why he was willing to mislead her," Dobbin told judges Timothy Holroyde and Judith Farbey during a preliminary appeal hearing.

Assange supporters protest in London ahead of first hearing in extradition appeal
Assange supporters protest in London ahead of first hearing in extradition appeal AFPTV / Caroline BRENIERE

"Experts aren't allowed to mislead for any reason."

Judge Baraitser concluded that Assange's mental health would deteriorate faced with the "harsh conditions" likely to await him in the US jail system, "causing him to commit suicide".

Assange, who is currently being held at London's high-security Belmarsh Prison, is wanted on 18 charges in the US relating to the 2010 release by WikiLeaks of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq.

"This is a mockery of liberty," Colombian protester Mario Mantilla told AFP
"This is a mockery of liberty," Colombian protester Mario Mantilla told AFP AFP / JUSTIN TALLIS

The 50-year-old Australian followed court proceedings via a video link from prison, sporting long white hair and an unbuttoned white shirt with an untied tie draped around his neck.

Dozens of his supporters gathered outside the court, some waving placards saying "Free Assange"" and "10 years enough".

"This is a mockery of liberty," Colombian protester Mario Mantilla, 58, who was dressed in a Statue of Liberty costume splashed with fake blood, told AFP.

"He is a journalist whose only crime is spilling the rotten, dirty business of the world," he added.

Washington claims Assange helped intelligence analyst Chelsea Manning steal the 2010 documents before exposing confidential sources around the world.

After Sweden first issued an arrest warrant for Assange in 2010 over allegations of sexual assault, he sought asylum in Ecuador's embassy in London, where he remained from 2012 until 2019.

In April 2019, Ecuador, by then ruled by right-wing President Lenin Moreno, revoked his citizenship. British police dragged Assange out of the embassy.