The 49-year-old Australian, who is currently being held on remand at a high-security jail, faces 18 counts from US prosecutors that could see him jailed for up to 175 years.
The hearing at the Central Criminal Court, the Old Bailey, is due to last three to four weeks. It had been due to go ahead in April but was delayed due to the coronavirus outbreak.
Any ruling is “almost certain” to be appealed by the losing side, according to John Rees, of the Don’t Extradite Assange Campaign, raising the prospect of more time behind bars for the former hacker.
Rees told AFP that Assange — who has become a figurehead for press freedom and investigative journalism — had a “very strong defence” but was concerned the case was “highly politicised”.
A previous hearing in February was told that US President Donald Trump had promised to pardon Assange if he denied Russia leaked emails from the campaign of Hillary Clinton, Trump’s opponent in the 2016 election.
Assange faces charges under the US Espionage Act for the 2010 release of 500,000 secret files detailing aspects of US military campaigns in Afghanistan and Iraq.
Washington claims he helped intelligence analyst Chelsea Manning to steal the documents before recklessly exposing confidential sources around the world.
At the February hearing, Assange’s lawyer, Edward Fitzgerald, said his client would not get a fair trial in the United States and would be a suicide risk.
James Lewis, representing the US government, said WikiLeaks was responsible for “one of the largest compromises of classified information in the history of the United States”.
“Reporting or journalism is not an excuse for criminal activities or a licence to break ordinary criminal laws,” he added.