Julian Assange a no-show as critical court case gets underway in ...

Protesters chanted and waved placards with his face on them outside, but Julian Assange was a no-show in court, as he tries to avoid extradition from Britain to the United States.

With legal options fast running out, Mr Assange — who is being held at London's Belmarsh Prison — had been granted permission to attend Tuesday's hearing at the Royal Courts of Justice in person.

However, when proceedings began, his barrister Edward Fitzgerald KC told the court the WikiLeaks founder was too ill to attend the hearing, even via video link from jail.

Last week, Mr Assange's wife, Stella described his health as "in decline, physically and mentally".

The Australian is asking the High Court of England and Wales permission to appeal a decision that would see him handed to American authorities.

He is facing extradition to the US, where he has been charged with 18 criminal offences, including obtaining, receiving and disclosing classified information.

A large crowd can be seen outside the Royal Courts of Justice in London before Julian Assange's appeal application began.(ABC News: Adrian Wilson)

The charges relate to material published on Mr Assange's WikiLeaks website in 2010.

That information — given to him by soldier-turned-whistleblower Chelsea Manning — detailed evidence of, amongst other things, war crimes committed by US forces in Iraq and Afghanistan.

Mr Assange was living in the United Kingdom at the time of publication, and this hearing represents his final legal avenue in that jurisdiction to avoid the extradition.

If he is successful, he will remain is custody while the High Court hears an appeal at a later date.

If he is not, Mr Assange could try and take his case to the European Court of Human Rights — although legally, it would be possible for British authorities to send him to the US before that.

A woman outside the Royal Courts of Justice, in London on Tuesday morning.(ABC News: Adrian Wilson)

Mr Fitzgerald KC on Tuesday told the court all the information his client had published was in the public interest and that the offences Mr Assange had been charged with were political, something which would make him exempt from extradition under treaty that exists between the UK and US.

He said Mr Assange had effectively become the "spokesperson for a global political movement against the United States", which amounted to being "almost like an opposition party".

Mr Fitzgerald told the court the "revelations" Mr Assange had published changed government policy in the US.

The court is hearing arguments from Mr Assange's legal team on Tuesday, before lawyers for the US state their case on Wednesday.

Lawyer Jennifer Robinson (left), Stella Assange and WikiLeaks editor-in-chief Kristinn Hrafnsson arrive at court on Tuesday.(Reuters: Hannah McKay )

Mr Assange has been behind bars since April 2019 when London's Metropolitan Police arrested him inside the Embassy of Ecuador, on Kings Cross Road, where he had resided for almost seven years while claiming political asylum.

The 52-year-old's legal team argue the US Espionage Act 1917, under which 17 of Mr Assange's 18 charges fall, has never been used to prosecute publishers before.

They also argue extraditing Mr Assange, who is not an American citizen and was not living or working in the US at the time of the alleged offences, would represent an unbridled extension of Washington's judicial reach.

The Australian government and opposition are both against the extradition, with Prime Minister Anthony Albanese last week telling federal parliament he wanted the process resolved "amicably".

Manning was released from custody in 2017 after having her 35-year prison sentence commuted to seven years by then-US President Barack Obama.

The hearing continues.

Julian Assange had plenty of support outside the Royal Courts of Justice on The Strand.(ABC News: Adrian Wilson)