The 49-year-old Australian appeared in court on Sept. 7 clean shaven with spectacles perched in his short cropped hair.
He wore a smart dark suit, maroon tie and white shirt.
Assange spoke to confirm his name and date of birth at the start of the hearing.
He formally said he did not consent to extradition, following a fresh indictment lodged in the U.S.
Assange has been in high-security Belmarsh Prison for 16 months and is wanted in the U.S. over the publication of hundreds of thousands of classified documents in 2010 and 2011.
He is 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 U.S. Department of Defence computer.
If convicted, he faces a maximum possible penalty of 175 years in jail.
Assange’s supporters have accused the U.S. administration of targeting Assange 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 Sept. 7, where a small stage has been erected for a planned protest.
Demonstrators roared in applause as speeches demanded the British government free Assange.
Earlier on Sept. 7 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 STG100,000 ($A182,000).
Speaking on Sept. 6, Moris, who has two young sons with Assange, described the possible impact on their family.
“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.
“Julian’s case has huge repercussions for freedom of expression and freedom of the press. This is an attack on journalism.”
It is expected dozens of witnesses will be called over four weeks.
Assange has been held on remand in Belmarsh 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.
By Emily Pennink, Henry Vaughan and Taz Ali