Around a thousand people formed a human chain around the UK parliament in London, demanding the release of WikiLeaks founder Julian Assange.
The Australian-born publisher has been held in a high-security London prison since 2019, and is currently appealing against a decision to extradite him to the United States.
He is wanted by US authorities to face trial for allegedly violating the US Espionage Act by publishing classified military and diplomatic files in 2010, related to the wars in Afghanistan and Iraq.
The 51-year-old could face decades in jail if found guilty.
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.
His case has become a cause celebre for media freedom and his supporters accuse Washington of trying to muzzle reporting of legitimate security concerns.
Supporters in London, including Jeremy Corbyn, the former leader of the British Labour Party, held life-size cut-outs of Assange, along with posters reading "Free Assange, end extradition", "journalism is not a crime" and "prosecute war crimes, not Assange".
The chain formed outside the front of the Houses of Parliament spanned the River Thames over Westminster Bridge before winding itself along the river opposite the historic building.
Supporters of Assange were also due to protest outside the US Department of Justice in Washington and another protest took place in Zagreb, Croatia.
