Assange's supporters hail him as an anti-establishment hero and a journalist who is being persecuted for exposing US wrongdoing.

At the start of what could be his last chance to stop his extradition from the UK to the US at London's High Court, Assange's lawyers and wife said the case was politically motivated and an attack on all journalists.

Stella Assange likened his case to that of Alexei Navalny, the Russian opposition activist who died in prison on Friday while serving a three-decade sentence.

"Julian is a political prisoner and his life is at risk. What happened to Navalny can happen to Julian," she told reporters outside court where a large crowd called for his release.

Assange himself was not in court nor watching remotely because he was unwell.

The Australian's legal battles began in 2010 and he subsequently spent seven years holed up in Ecuador's embassy in London before he was dragged out and jailed in 2019 for breaching bail conditions.

He has been held in a maximum-security jail in London ever since, even getting married there, while the UK finally approved his extradition to the US in 2022.

His legal team is trying to overturn that approval at a two-day hearing.

Their argument is that previous judges failed to address their case that the extradition was politically-motivated and a deliberate attempt to punish and silence him for exposing US "state-level crimes".

"Mr Assange is being prosecuted for engaging in ordinary journalistic practices of obtaining and publishing classified information which is true and of public interest," Edward Fitzgerald, Assange's lead lawyer, told the court.

He said, if convicted, Assange could be given a sentence as long as 175 years but likely to be at least 30 to 40 years.

His colleague Mark Summers said there was evidence there had been a "truly breathtaking plan" to kidnap or murder Assange while he was in the Ecuador's embassy, and former US president Donald Trump had asked for "detailed options" to kill him.

In 2021, Yahoo News reported CIA officials had drawn up options for Trump's administration for dealing with Assange while he was in the London embassy.

"Senior CIA officials requested plans, the president himself requested on being provided with options on how to do it and sketches were even drawn up," the lawyer said on Tuesday.

In their written submissions, lawyers for the US government said their case against him was "consistently and repeatedly misrepresented" by Assange's legal team.

They said he was not being prosecuted for publication of the leaked materials but for aiding and conspiring with former US army intelligence analyst Chelsea Manning to unlawfully obtain them, then disclosing names of sources and "putting those individuals at grave risk of harm".

If Assange wins this case, a full appeal hearing will be held to again consider his challenge.

If he loses, his only remaining option would be at the European Court of Human Rights (ECHR) and Stella Assange said his lawyers would apply to the European judges for an emergency injunction if necessary.

WikiLeaks first came to prominence in 2010 when it published a US military video showing a 2007 attack by Apache helicopters in Baghdad that killed a dozen people, including two Reuters news staff.

It then released thousands of secret classified files and diplomatic cables that laid bare often highly critical US appraisals of world leaders from Russian President Vladimir Putin to members of the Saudi royal family.

Assange's supporters include Amnesty International, media groups and politicians including Prime Minister Anthony Albanese, who last week voted in favour of a motion calling for his return to Australia.