Hundreds of protesters including Jeremy Corbyn, the former leader of the United Kingdom's opposition Labour Party, gathered in a line which stretched from parliament's perimeter railings and snaked across nearby Westminster Bridge to the other side of the River Thames.

Stella Assange, who is married to the Australian-born activist, said the British government should speak to authorities in the US to end the extradition bid which was launched in 2019.

"It's already gone on for three-and-a-half years. It is a stain on the United Kingdom and is a stain on the Biden administration," she said.

Corbyn urged other politicians to back Assange's cause.

"I would say to MPs - actually of any party - you're there to represent democracy and rights," the now-independent MP told the PA news agency.

"That's what you sign up for... if Julian Assange is extradited, it will set forth fear among other journalists of doing anything to expose truth.

"It becomes a self-censorship of journalists all around the world. They'll say, 'hang on, I'm not touching that, look what happened to Julian Assange'."

Supporters of Assange were also due to protest on Saturday outside the US Department of Justice in Washington DC.

Assange, 51, is wanted by US authorities on 18 counts, including a spying charge, relating to WikiLeaks' release of confidential US military records and diplomatic cables.

The US says he put lives in danger.

His supporters say he has been victimised because he exposed US wrongdoing in conflicts in Afghanistan and Iraq.

Assange's legal team have lodged an appeal at the High Court against the UK's decision to extradite him.

with PA

Australian Associated Press