Julian Assange was strip searched and moved to a bare cell on the day the Home Secretary announced she was allowing his extradition to the United States, his wife said.
Stella Assange said the WikiLeaks founder was told he was being moved to a bare cell “for his own protection”.
He had no visits over the weekend following the announcement last Friday.
“Imagine what it’s like for him to have to process this alone,” said Mrs Assange.
John Rees, a leading member of the campaign for Mr Assange to be freed, told the PA news agency: “This is simply extrajudicial punishment.
“It’s unacceptable and it’s surely illegal. But it shows how much pressure the authorities are under to free Assange that they behave this vindictively.
Supporters including journalist John Pilger also criticised the treatment of Mr Assange on the day Hone Secretary Priti Patel made her announcement.
Mrs Assange told the PA news agency: “Prison is a constant humiliation but what happened on Friday felt especially cruel.
“After the announcement of Patel’s decision, Julian was taken from his cell so that he could be strip searched, and then escorted to a bare cell where he remained for the rest of the weekend.
“His own cell was searched. They were looking for things that could be used to take one’s own life.
“In the bare cell, guards logged his status every hour until he was allowed to return to his cell on Tuesday.
“This kind of thing never becomes more tolerable. Any person would find it degrading. The mental strain on Julian is enormous as it is, having to process what is essentially a death sentence.
“The fact he is imprisoned while this outrageous extradition proceeds is a grave injustice in itself. He needs to deal with all that, while preparing for a complex appeal to the High Court.”
Labour MP Richard Burgon said: “Appalling news that Julian Assange was strip searched and moved to a bare cell on the day Priti Patel agreed his extradition.
“It’s cruel, degrading and deliberately humiliating. This kind of treatment seems to be designed to break him for his role in exposing US war crimes.”