The Courage Foundation, which offers legal support for whistleblowers and journalists, said Mr Assange had become “isolated” inside the Ecuadorian embassy in London, with “severe restrictions” on his communications and visitors.
The organisation said his expulsion from the embassy may be imminent, putting him at risk of extradition to the United States.
The foundation said in a statement: “The US government has pursued a Grand Jury investigation into WikiLeaks since 2010, with major politicians and public figures calling for Assange’s arrest or execution.
“It has recently been confirmed that the US government has already charged Assange in a sealed indictment and seeks his arrest and extradition.
“In response to these developments, the Courage Foundation is launching a Julian Assange Legal Defence Fund.
“Prosecuting Assange for publishing constitutes a major attack on press freedoms at large and his transatlantic legal defence will require contributions from supporters around the world.”
The Courage Foundation said the US government would use all its resources to force Mr Assange’s extradition from the UK and take him into custody.
Mr Assange has been living inside the embassy for more than six years.