LONDON — Four months after Britain went into lockdown, most office workers have yet to return to the City of London. The once heaving thoroughfares of this global financial hub, also known as the “Square Mile,” have remained largely empty since March.

But just after 8 a.m Tuesday, a yelling scrum of photographers, reporters and protesters spilled across the sidewalks and into the road outside London’s central criminal court, the Old Bailey, to watch a 79-year-old woman dressed in a yellow trouser suit and baseball cap suspend herself inside a bird cage 10 feet in the air, squawking at the top of her lungs.

“I am the canary in the coal mine,’ shrieked Dame Vivienne Westwood, the flamboyant British fashion designer, couturier to everyone from supermodels to world leaders, punk icon, eco-warrior and political activist. She held a megaphone aloft and said to the cheering crowd: “If I die down the coal mine from poisonous gas, then that’s the signal.”

Ms. Westwood had been lured out of 16 weeks in isolation by the plight of the WikiLeaks founder Julian Assange, who is fighting extradition from Britain to the United States.

Mr. Assange, 49, is wanted by U.S. authorities to stand trial on 18 charges, including conspiring to hack government computers and espionage. Last year, the United States began extradition proceedings after he was dragged from the Ecuadorean embassy in London, where he had been holed up for almost seven years.

“I am Julian Assange,” Ms. Westwood continued, legs swinging in the breeze, as a garbage truck pulled over and started to reverse loudly down a small side street. Several bemused members of the court staff peered through the Old Bailey’s large archways to get a look at the unfolding commotion, while a white van driver tooted his horn in appreciation.

“And I am a canary. I am half poisoned already from government corruption and gaming of the system and legal system by governments,” Ms. Westwood said. The designer — who used salty language throughout her speech — said she was “still whistling away” while the world’s 7 billion people did not know what was going on.

Ms. Westwood, who made her name by defining the rebellious aesthetic of London in the 1970s, has dressed the Sex Pistols, supermodels like Naomi Campbell and Kate Moss, and celebrities like Harry Styles and Helena Bonham Carter, translating the rigor and shock value of punk music into more commercially palatable tartan offerings and iconoclastic ball gowns with safety pins, tulle and slogans.

She is no stranger to headline-grabbing stunts, from dressing up as Margaret Thatcher for a Tatler cover in 1989, accepting an Order of the British Empire from Queen Elizabeth II at Buckingham Palace with no panties on and driving a tank to then-Prime Minister David Cameron’s Oxfordshire home in an anti-fracking protest in 2015.

Mr. Assange first made headlines in 2010 when he began publishing secret American military and diplomatic documents that were provided by the former Army intelligence analyst Chelsea Manning, who was convicted at a court-martial in 2013 of leaking the documents.

For the last year Mr. Assange has been held at Belmarsh Prison in London, and, if he is successfully extradited, he could face as many as 175 years in prison if found guilty on all charges.

After several minutes on the megaphone and then being carefully winched to safety, Ms. Westwood pretended to be a bird by screeching at her cage for photographers as protesters held up “Free Assange” banners nearby. Later, she explained that her activist son, Joe Corré, a “captain” of a campaign in defense of Mr. Assange, was the mastermind behind the protest.

Mr. Corré, who founded the underwear label Agent Provocateur, turned down a Member of the Order of the British Empire honor in 2007 in a protest against Britain’s participation in the Iraq war.

“There’s no time to spare now whatsoever,” Ms. Westwood said as she pulled on a face mask. “If Julian gets sent to America, it is the worst thing that could happen in the world for justice and freedom of speech. This could happen to every journalist.”

Ms. Westwood also said that, despite being a designer in business for more than 50 years, she hadn’t spent that much time fashioning her canary outfit.

“It was the only thing I could find that was yellow, though I did try hard to make my eyes look like that of a bird — can you see,” she said, widening them to show off the wild multicolored plumes of crayon swirls that swept up her temples. “Are you looking closely enough?”