Assange's brother, Gabriel Shipton, told Reuters that the Australian publisher's legal team appealed his extradition, which was formally approved by U.K. Home Secretary Priti Patel last month.

"We also urge the Australian government to intervene immediately in the case to end this nightmare," Shipton said.

Supporters of Assange held protests ahead of his 51st birthday on Saturday, including one in an open-top double-decker London tour bus that passed by British government buildings in Westminster on Friday. One of the demonstrators, 79-year-old Gloria Wildman, told Agence France-Presse that Assange has "been in prison for telling the truth."

"If Julian Assange is not free, neither are we; none of us is free," she added. Myriad human rights, journalistic, and other groups have condemned Assange's impending extradition and the U.S. government's targeting of a journalist who exposed American war crimes. In a Thursday statement, the Australian Journalists Union said that "the charges against Assange are an affront to journalists everywhere and a threat to press freedom."

Assange—who suffers from physical and mental health problems including heart and respiratory issues—faces U.S. charges including Espionage Act violations for which he faces up to 175 years behind bars if fully convicted.

Among the classified materials published by WikiLeaks—many provided by whistleblower Chelsea Manning—are the infamous "Collateral Murder" video showing a U.S. Army helicopter crew killing a group of Iraqi civilians, the Afghan War Diary, and the Iraq War Logs,  which revealed American and allied war crimes.