"He truly represents the very core value of why we are fighting, the freedom of the press."
Mr Assange faces 18 charges in the US relating to the 2010 release by WikiLeaks of 500,000 secret files detailing aspects of military campaigns in Afghanistan and Iraq.
Evidence in the trial stage of his long-running battle against extradition entered its fourth week on Monday and is due to end within days.
But the judge hearing the case has said it will be at least six weeks before she delivers a verdict - meaning it will come after the US presidential election on 3 November.
Chinese artist Ai Weiwei joins silent protest against Assange extradition
It is the latest in a series of legal battles faced by Mr Assange since the leaks a decade ago.
In 2010, he faced allegations of sexual assault and rape in Sweden, which he denied.
He was in Britain at the time but dodged an attempt to extradite him to Sweden by claiming political asylum in Ecuador's embassy in London.
In November 2019, Swedish prosecutors dropped their investigation over the rape allegation, after failing for years to interrogate Mr Assange in person while he was holed up in the embassy.
Mr Assange lived in a small apartment there for seven years, but was turned over to British police in April 2019 after a change of government in Quito.
Mr Ai, 62, said he had met Mr Assange several times during this period and, although he did not know him well, felt he was being unfairly targeted.
"Each step is more and more difficult for him. It's unbelievable. He is prepared to fight but alas it's not fair conditions, this is not fair to him.
"They tried to smash him and his name," he added.
Mr Ai said he chose to stage a silent protest because "all the words we want to say are already there".
"As an artist, if I cannot use my art - which is very limited - then I rather just be silent."
Mr Ai, a long-time critic of the government in Beijing, is China's best known modern artist.
He helped design the Bird's Nest Olympic stadium, but was detained for 81 days in 2011 as part of a crackdown on dissent.
His passport was confiscated and only returned to him in 2015, after which he moved to Berlin.