The US government appeal case against a decision that blocks the extradition of WikiLeaks' co-founder Julian Assange began on Wednesday.
The US government's legal representatives will try to overturn the decision, which was issued last January, based on the concerns over Assange's mental health.
The US said it was "extremely disappointed" with the decision blocking his extradition.
If the appeal from the US is successful, the case will go back to a lower court for a new decision. The losing party has a right to a final appeal to the UK's Supreme Court.
Earlier this week, the US administration was urged to drop all charges against Assange.
Speaking at a news conference in London, alongside WikiLeaks' editor-in-chief Kristinn Hrafnsson and Assange's partner Stella Moris, Rebecca Vincent from media advocacy group Reporters Without Borders said now was a good time for US President Joe Biden to "distance himself" from his predecessor's policies.
The Central Criminal Court, otherwise known as the Old Bailey, delivered its judgment in January and said Assange cannot be extradited for concerns related to his mental health.
"The overall impression is of a depressed and sometimes despairing man fearful for his future," Judge Vanessa Baraitser had said.
Assange will face 18 counts of hacking the US government computers and violating the country's espionage law if he is extradited. This can involve a potential prison sentence of 175 years, depending on the appeal.
The appeal hearing will begin at the Royal Courts of Justice on Wednesday.
The US is accusing Assange of espionage after WikiLeaks published hundreds of thousands of pages of government documents, emails, and other communications, including US troops' war crimes in the Afghanistan and Iraq wars in 2010 and 2011.
Prosecutors argued previously that Assange assisted US Army defense analyst Chelsea Manning, but the 49-year-old WikiLeaks co-founder denied the allegations.
He was dragged out of Ecuador's embassy building in London in 2019, where he had taken refuge for more than seven years.
The British police said he was arrested for skipping bail in 2012 and on behalf of the US due to an extradition warrant.
Later, he was found guilty of breaking his bail terms in 2012 after failing to surrender to security services by the Westminster Magistrates' Court and given a 50-week prison term.
But, Assange was not released from custody due to "substantial grounds" that he would abscond.