Campaigners and supporters will march through central London as Assange faces extradition to the United States, where he fears being imprisoned for the rest of his life.
He has been held in London’s high-security Belmarsh Prison for more than four years as he fights extradition.
Speakers at the Parliament Square event include Assange’s wife, Stella, alongside a sculpture called Anything To Say, which features life-sized bronze figures of whistleblowers Edward Snowden, Assange and Chelsea Manning, each standing on their own individual chair.
Adjacent to them is an empty chair, representing the general population, encouraging everyone to “stand up instead of sit like the others”.
The sculpture has been displayed in Germany, Switzerland, France, Italy, Belgium, Serbia and Australia.
Author and broadcaster Charles Glass conceived the idea, brought to life by Italian artist Davide Dormino.
“As an artist, I feel I have a duty to defend freedom of speech and the right to know,” Dormino said.
“That is why I have created an empty chair, which allows us to stand taller and raise ourselves.
“It changes our perspective and prompts us to look beyond what we are shown and what is hidden.”