Dressed in a canary yellow outfit, the 79-year-old maven posed in a giant bird cage and led protesters in a “Free Julian Assange” chant outside London’s Central Criminal Court.
Assange is due to appear for an extradition hearing on Sept. 7.
If extradited to the U.S., he will face charges of conspiracy to commit computer intrusion. Prosecutors say he damaged national security by publishing hundreds of thousands of classified documents, including diplomatic cables and military files on the wars in Iraq and Afghanistan, that harmed the U.S. and its allies and aided its adversaries.
The 49-year-old is currently being held in a high-security prison in London.
Assange was arrested last year after being evicted from the Ecuadorian Embassy in London, where he had been holed up for years.
He maintains he was acting as a journalist entitled to First Amendment protection. His lawyers have argued the U.S. charges of espionage and computer misuse were politically motivated and an abuse of power.
Westwood told the Associated Press after her protest that freeing Assange would mean that “journalists can continue to tell the truth, rather than repeating the spin that’s dealt to them by the government.”
The designer has turned to activism in recent years, leading campaigns in support of Assange and drawing attention to climate change at her fashion shows.
The Associated Press contributed to this report.