The 49-year-old, who has been in high-security Belmarsh Prison for 16 months, is wanted over the publication of hundreds of thousands of classified documents in 2010 and 2011.
He is expected to appear in court in person on Monday for the first time in months, facing 18 charges – including plotting to hack computers and conspiring to obtain and disclose national defence information.
The allegations include that Assange conspired with army intelligence analyst Chelsea Manning to crack a scrambled password, known as a “hash”, to a classified US Department of Defense computer.
If convicted, he faces a maximum possible penalty of 175 years in jail.
Assange’s supporters have accused the US administration of targeting the Australian national for “political” reasons after WikiLeaks exposed alleged war crimes and human rights abuses.
Dozens of supporters, including his father, John Shipton, gathered outside the Old Bailey on Monday, where a small stage has been erected for a planned protest.
Demonstrators roared in applause as speeches demanded the government free Assange, while some banged drums and carried banners.
A mobile billboard van drove past featuring a “Don’t extradite Assange. Journalism is not a crime” slogan and a picture of his face.
Supporter Lise Brand, 56, from Surrey, said: “He is looking at 175 years in an American prison, I don’t hold any hope.
“The UK government and justice system has treated him so badly.”
Meanwhile, Assange’s partner Stella Moris arrived at Downing Street in a bid to deliver a Reporters Without Borders petition against the extradition, which has been signed by around 80,000 people.
Last month, she launched a Crowdjustice campaign to help fund his defence which has now topped £100,000.
Speaking on Sunday, Ms Moris, who has two young sons with Assange, described the possible impact on their family.
She said: “To the boys, Julian has become a voice on the telephone, not their father whom they can see and hug.
“It is heartbreaking to think that if Julian is extradited and put in a US super-max prison, the boys will never get to know their father and he will never see them grow up.
“That is what is at stake for us as a family. But there are also much bigger issues that we are fighting for.
“Julian’s case has huge repercussions for freedom of expression and freedom of the press. This is an attack on journalism.
“If he is extradited to the US for publishing inconvenient truths about the wars in Iraq and Afghanistan then it will set a precedent and any British journalist or publisher could also be extradited in the future.”
The extradition case, which was delayed due to the coronavirus pandemic, is being heard by District Judge Vanessa Baraitser at the Old Bailey.
It is expected that dozens of witnesses will be called to give evidence over four weeks, with the judgment likely to be delivered at a later date.
Assange’s legal team is being spearheaded by Edward Fitzgerald QC, with James Lewis QC acting for the US authorities.
No journalists are allowed in court for the hearing because of Covid-19 restrictions, with some observing by videolink from another room in the building and others provided with a remote link.
Assange has been held on remand in Belmarsh prison since last September after serving a 50-week jail sentence for breaching his bail conditions while he was in the Ecuadorian embassy in London for almost seven years.