Retired Swedish appeal court judge Brita Sundberg-Weitman launched an outspoken attack on the authorities responsible for an arrest warrant.
She was flown to London by Assange’s legal team to give evidence supporting their argument that extraditing him would be a “flagrant denial of justice”.
Speaking in the witness box at Woolwich Crown Courtshe said she could not understand the “attitude” of prosecutor Marianne Ny.
Mrs Sundberg-Weitman, an associate professor at Stockholm University, accused Ms Ny of having a “rather biased view against men”.
She added: “She seems to take it for granted that everybody under prosecution is guilty.
“I think she is so preoccupied with the situation of battered women and raped women that she has lost balance.”
The claims were made on the first day of a two-day hearing that will decide whether Assange, a 39-year-old Australian, should be sent to Sweden.
He is wanted by police over allegations he sexually assaulted one woman and raped another during a visit to Stockholm last August.
Assange’s legal team argue extradition would breach his human rights and fear he could ultimately be taken against his will to the United States and executed.
But a barrister representing the Swedish authorities said the fears are unfounded, the proper legal process is being followed and Assange has a case to answer.