Julian Assange's father John Shipton hopes an "upwelling of support" for the WikiLeaks founder will move the British government to intervene and stop his extradition to the US.
About 500 people rallied in London on Saturday in support of Assange, whose extradition hearing is due to start on Monday.
Pink Floyd's Roger Waters, The Pretenders' Chrissie Hynde and Brian Eno joined Greek politician and writer Yanis Varoufakis, British Iraqi rapper Lowkey and fashion designer Vivienne Westwood at the protest at Parliament Square.
Mr Shipton hopes the support will encourage the UK government to intervene in the case.
"What I perceive is an upwelling amongst the people of support - a historical moment of change that will ensure the government of the United Kingdom can act firmly in their own interests," he told AAP.
"So that the ... plague of malice that emanates from the Crown Prosecution Service, and brings into disrepute the administration of law in the United Kingdom, can be stopped and Julian's freedom can come."
Rapper Lowkey lashed out at UK newspapers, which he accused of gaining advertising sales by publishing WikiLeak's disclosures but failing to publicly defend Assange.
"How many oil adverts were put on the pages of The Guardian on the back of Julian Assange's stories and works for WikiLeaks?" he told the crowd.
"How much money did these same newspapers that are hanging him out to dry, that are clamouring for his execution, how much money have they got from major oil companies that advertise on their pages?"
Mr Varoufakis, who plans to visit Assange in London's Belmarsh prison on Sunday, urged ralliers not to let the WikiLeaks founder turn into "a latter-day Man in the Iron Mask".
"We are in the business of allowing unalloyed, unarmed truth to have the final word, we are in the business to ensure Julian's sentence will end neither with a bang or with a whimper, but with a magnificent full stop," he said.
Assange will on Monday begin his legal battle against extradition to the US, where he faces 17 spying charges and one charge of conspiring to commit computer intrusion - charges that carry a total prison sentence of 175 years.
They relate to WikiLeaks' 2010 publication of thousands of classified Pentagon files on the Iraq and Afghanistan wars - some that revealed war crimes and the torture of prisoners - along with US diplomatic cables.
Australian Associated Press