The boxing champion - who has confirmed being sectioned twice over his ongoing battle with mental health this year - said that he wasn't scared during the ordeal back in April.
"I was sitting in the dining area when I saw a guy out the corner of my eye," he told The Mirror.
"He'd grabbed a knife out the kitchen and was coming at me when security staff bundled him over. I froze in shock at what was happening. A lot of the people in these hospitals are very sick and need help.
"I didn't feel scared. How can you feel frightened of anything when you have been in the ring with Mike Tyson twice? But it did shock me. After it happened, getting out became even more important to me."
Bruno was taken to a specialist mental health unit in Basildon, Essex on April 13, before getting released the following week.
Talking about his time at the hospital, he said: "They took me to a place which was like a living hell... Being in that hospital was worse than being in prison. At least in prison you know why you are there. But I could never understand why I was back in the system.
"My room was no bigger than a prison cell. All I had was a bed. There was no TV. All I did was listen to my iPod to try to calm me down. There was a tiny window in the room but you couldn't open it. I wasn't even allowed to go to the toilet without being supervised. It was degrading.
"Going back into hospital was not my decision. But once it happened I took my punishment and I didn't cry. I was just pumped full of drugs and it didn't make me better, it made me worse."
The sportsman added that he is determined not to return, stating: "I was in there five weeks but it felt like five years. It is harder than being in the ring. At least in boxing it is 12 rounds and then you come out. In hospital I could never see when it was going to end.
"Now I am out I want to stay out. I take my medicine and I am doing what the doctors say."
Frank Bruno was previously sectioned under the Mental Health Act 1983 in 2003, before later being diagnosed as bipolar.
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