The doctors urged Britain to end the "psychological torture and medical neglect of Julian Assange … before it's too late."
"Should Assange die in a UK prison, as the U.N. special rapporteur on torture has warned, he will have effectively been tortured to death," the Doctors for Assange group said in a letter to medical journal The Lancet.
"Much of that torture will have taken place in a prison medical ward, on doctors' watch," they said.
"The medical profession cannot afford to stand silently by, on the wrong side of torture and the wrong side of history, while such a travesty unfolds."
Australian citizen Assange, 48, is due to face an extradition hearing in London next week, after the U.S. government accused him of conspiring with former U.S. military intelligence analyst Chelsea Manning to leak a trove of classified material in 2010.
The documents relate to the wars in Iraq and Afghanistan and to information on intelligence sources.