Nazanin Zaghari-Ratcliffe, the British-Iranian woman jailed in Iran for alleged spying, was kept in solitary confinement and chained to a bed in a psychiatric ward in Tehran, her husband has said.
Richard Ratcliffe said his wife was returned to prison after being discharged from hospital on Saturday.
He says she described her treatment on the ward as "proper torture".
Last month she went on hunger strike for 15 days to protest her detention.
It comes amid escalating tensions between the UK and Iran over the seizure of oil tankers.
In a press release, the Free Nazanin Campaign said Mrs Zaghari-Ratcliffe, 40, told her husband she was "broken" by the experience.
Speaking to him on the phone on Sunday and Monday, he said she told him: "They did all they could to me - handcuffs, ankle cuffs, in a private room two by three metres, with thick curtains, and the door closed all the time.
"I wasn't allowed to leave the room, as I was chained to the bed. It was proper torture. It was tough, and I was struggling."
He said she continued: "I never thought I would end up there. I always found myself strong, and then finding myself there - it was really traumatising.
"There was no justification for it. I am cross at them. I am not scared. The amount of scars I got. I have been put through hell."
She was transferred back to Evin prison after breaking out of her bindings and telling security guards she was at risk of self-harming if she had to stay in the hospital, Mr Ratcliffe said.
Speaking before her return to jail, he said he felt "euphoric" when he heard his wife had been moved to a hospital, thinking it could be a prelude to having treatment or even to her release.
However, after her father was refused access to visit her in hospital or allowed to speak to her on the phone, the family grew increasingly concerned.
Her admission to the mental health unit came after she went on hunger strike for 15 days last month in protest against her "unfair imprisonment".
Mr Ratcliffe joined her protest, camping on the pavement outside the Iranian Embassy in London and not eating.
In November 2017, then Foreign Secretary Boris Johnson faced criticism for suggesting Mrs Zaghari-Ratcliffe was training journalists while in Iran - remarks he later apologised for and clarified, saying he had no doubt she was on holiday there.
Former Foreign Office minister Sir Alan Duncan, who resigned on Monday over the prospect of Boris Johnson as prime minister, said in his resignation letter he was "deeply upset" that discussions about her possible release had come to "an abrupt halt" during his time in government.
Earlier this year, Foreign Secretary Jeremy Hunt granted her diplomatic protection in a bid to resolve her case.
Labour MP Tulip Siddiq, who is Mrs Zaghari-Ratcliffe's MP, last week questioned whether the detention of the Iranian Grace 1 oil tanker by Royal Marines was linked to the developments in her case.
In response, Foreign Office minister Andrew Murrison said: "I don't believe the two are directly linked."
Mrs Zaghari-Ratcliffe was arrested at Tehran's Imam Khomeini Airport in April 2016 and has always said the visit was to introduce her daughter to her relatives.