The Australian activist and Wikileaks founder is awaiting deportation to the United States from HMP Belmarsh, where he fears he could face a 175-year prison sentence.
Mr Assange’s wife, Stella, accompanied by their two young sons, were cheered on by protesters forming a human chain that snaked along Westminster Bridge, along the Houses of Parliament facade and into the gardens of the Victoria tower.
One of the boys smiled at the reception from the crowd, some of whom were carrying life-size cardboard cutouts of their father.
Mr Corbyn, who has linked arms with Leicester East MP Claudia Webbe and former union boss Len McCluskey, urged other politicians to support Mr Assange’s cause.
“I would say to MPs – in fact from any party – that you are here to represent democracy and rights,” the independent MP told the PA news agency.
“That’s what you’re signing up for…if Julian Assange is extradited, it will scare other journalists into doing anything to expose the truth.
“It becomes a self-censorship of journalists around the world. They’ll say, ‘wait, I’m not touching that, look what happened to Julian Assange’.”
While most protesters lined up outside Parliament, others marched through the streets chanting “Free Julian Assange” to the tune of Nelson Mandela from The Specials.
Brand made a brief appearance, holding Ms Assange’s hand as the couple were surrounded by reporters.
Mr McCluskey, after tying a yellow #FreeAssange ribbon into a makeshift scarf, claimed the US government was making an example of the imprisoned journalist.
The former Unite boss, who led Britain’s second biggest union until last year, said: ‘This is a global elite who are making it clear no one will put them under surveillance .
“They want to rule the world without anyone challenging what they’re doing, and that’s all Julian has done.
“He told the truth, and the truth was too much for them to swallow.”
A row broke out later in the day as stewards tried to move protesters through Victoria Tower Gardens after the human chain was broken.
“They are not real stewards, they are not real stewards,” insisted a man in an orange jumpsuit, while other activists refused to leave their posts.
“I’m not doing this for fun,” protested a steward before leaving.