Meanwhile, the court heard that Assange, if convicted in the US, could end up spending the rest of his life imprisoned in the federal Supermax prison in Florence, Colorado.

The facility is home to Unabomber Ted Kaczynski, Mexican drug lord Joaquin "El Chapo" Guzman, 1993 World Trade Center mastermind Ramzi Yousef and Zacarias Moussaoui, the only man ever convicted in a US court for a role in the September 11 attacks.

Ai, who visited Assange at the Ecuadorian embassy in London where he was holed up for seven years until April 2019 and subsequently at Britain's Belmarsh high security prison, said the authorities have a responsibility to protect the freedom of the press.

"He is prepared to fight, but this is not fair to him," he said.

"Free him, let him be a free man."

US prosecutors have indicted the 49-year-old Assange on 17 espionage charges and one charge of computer misuse over WikiLeaks' publication of secret US military documents a decade ago.

The charges carry a maximum sentence of 175 years in prison.

As well as arguing that the extradition would pose a threat to Assange's life, his defence team say that Assange is a journalist and entitled to First Amendment protections for the publication of leaked documents that exposed US military wrongdoing in Iraq and Afghanistan.

"He truly represents a core value of why we are free - because we have freedom of the press.," Ai said.

"We need a lot of protesting, and it can take any form. I'm an artist, if I cannot use my art, it's very limited, then I'd rather just be silent."

Assange's father John Shipton was also protesting outside the court.

"Ai Weiwei is an artist of gigantic international standing, he stands alongside Julian to give the fight international meaning," he said.

Ai, 63, is one of the world's most successful artists, famous around the world for his installations of bicycles and sunflower seeds.

In his native China, he was alternately encouraged, tolerated and harassed, spending time in detention and being barred for years from leaving the country.

He was arrested at Beijing's airport in April 2011 and held for 81 days without explanation during a wider crackdown on dissent.

He is now based in Berlin and in the UK.

On Monday, the court heard from two witnesses who said Assange would face intolerable conditions if extradited.

It is widely expected that Assange would be moved to the pre-trial facilities at Alexandria Detention Center in Virginia if extradited.

Yancey Ellis, a Virginia-based lawyer who has had clients at the facility, said Assange would likely be put into an administrative segregation unit for months or even years that would be akin to "solitary confinement".

And Joel Sickler, who heads the Justice Advocacy Group, said Assange would face "no meaningful interaction" in pre-trial confinement in a cell the size of "a parking space".

Sickler, who has decades of experience in the field of the US prison system, also said Assange would face the "real risk" of special administrative measures, or SAMs, being imposed on him by the US Attorney General if convicted.

The imposition of such measures could further curtail Assange's links and communications to the outside world as well as his movements within prison.

Sickler said it was highly likely that Assange could be sent to the Supermax facility in Colorado.

The prison, also known as ADX, is so secure, remote and austere it has been dubbed the "Alcatraz of the Rockies".

"Should Mr Assange be sent to ADX he will almost certainly spend all his time in ADX in solitary," Sickler said in written testimony to the court.

"While conditions at a penitentiary are severe, nothing compares to the near permanent solitary life of an inmate at ADX."

Assange's extradition hearing, which was delayed by the coronavirus pandemic, is due to end this week.

Australian Associated Press