“The evidence showed that the US was prepared to go to any lengths, including misusing its own criminal justice system, to sustain impunity for US officials in respect of the torture/war crimes committed in its infamous ‘war on terror’, and to suppress those actors and courts willing and prepared to try to bring those crimes to account,” he said. “Mr Assange was one of those targets.”

Fitzgerald told the court there was a real risk of further extrajudicial actions against him by the CIA or other agencies.

A key part of Assange’s case is also the extradition treaty between the UK and the US, which does not apply if the alleged wrongdoing is political. His legal team argued it was an abuse of process to seek extradition for a political offence.

Hundreds of protesters gathered outside the courts, tying golden ribbons with the words “Free Julian Assange now!” to the main fence as well as the surrounding gates and trees.

Several were waving Australian flags, holding placards with the words “Free Julian Assange” and “drop the charges”, and chanting: “There is only one decision – no extradition,” and, “US, UK, hands off Assange.”

James Lewis KC, representing the US government, said the appeal was an attempt to “reargue the findings of fact” made by a district judge who heard the extradition case in 2021.

Then-district judge Vanessa Baraitser also ruled that Assange should not be extradited, but authorities in the US subsequently brought a successful challenge against this decision.

Lewis, who is expected to make oral arguments on behalf of the US on Wednesday, added that Judge Baraitser ruled Assange was not being prosecuted for political reasons but “because he is alleged to have committed serious criminal offences”.

Assange’s lawyers say that if he is convicted of the US charges Assange could receive a prison term of up to 175 years.

At the rally outside the court his wife, Stella Assange, stopped and spoke to the crowd. “There is no possibility of a fair trial if he is extradited to the US,” she said. “He would never be safe.”

She said the case against her husband was an “admission by the US they now criminalise investigative journalism”.

“It is an attack on the truth and the public’s right to know,” she said, adding she believed what had happened to Alexei Navalny in recent days “can happen to Julian”.

Assange’s brother, Gabriel Shipton, said his sibling was going through an “immense amount of suffering” while being held in prison and said his health was “deteriorating”.

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“He’s suffering in there. He’s going through immense amount of suffering. He’s deteriorating, his health is in a very delicate position,” Shipton said before the hearing.

Assange has been held at London’s Belmarsh jail since April 2019 when he was sentenced for skipping bail conditions. He had previously spent seven years in the Ecuadorian embassy after breaking bail in 2012 when he was due to be extradited to Sweden on unrelated sexual assault charges, which were subsequently dropped. He was arrested and forcibly removed from the Ecuadorean embassy by British police in 2019.

The hearing, before Dame Victoria Sharp and Mr Justice Johnson,, concludes on Wednesday with their decision on whether Assange can bring the appeal expected at a later date.