On Thursday, Mr. Assange was to appear by video link from Belmarsh Prison in East London at a court hearing on the United States’ application for his extradition. But his British lawyer, Gareth Peirce, said her client was not well enough to participate. The presiding magistrate set June 12 for the next hearing and suggested it could be held in Belmarsh.

WikiLeaks said on Thursday that Belmarsh Prison authorities had moved Mr. Assange to its hospital wing after he experienced drastic weight loss and other health problems. It also said that when Mr. Assange’s Swedish lawyer, Per Samuelson, visited him in Belmarsh on Friday, he found “that it was not possible to hold a normal conversation with him.”

Mr. Melzer is not the first United Nations expert to criticize the conditions of Mr. Assange’s detention. The United Nations Working Group on Arbitrary Detention condemned the 50-week sentence for jumping bail as excessive and said that sending him to a high-security prison was akin to a conviction for a more serious crime.

Mr. Melzer said he had initially been skeptical about Mr. Assange’s case and had turned down a request from Mr. Assange’s lawyers in December to investigate. But he said that what he found after having accepted a second request from the lawyers in March, changed his mind.

“Wherever I delved into the case, I found a lot of dirty stuff,” he said in a phone interview.

In 20 years of working with victims of war, violence and political persecution, Mr. Melzer added in his statement, he had “never seen a group of democratic states ganging up to deliberately isolate, demonize and abuse a single individual for such a long time and with so little regard for human dignity and the rule of law.”

Mr. Melzer said that Britain should not extradite Mr. Assange to the United States or to any other country that did not provide reliable guarantees that it would not transfer him to the United States. He cited the treatment experienced by Chelsea Manning, the former Army intelligence analyst who sent WikiLeaks classified cables on events in Iraq and Afghanistan, as grounds for concern about the conditions in which Mr. Assange would be held.