Diddy's feeling a severe reversal of fortune right now -- locked up in a federal jail that's held plenty of famous people, but is also about as far from the lap of luxury as the embattled mogul's been in at least 30 years.

On the heels of the judge ordering Diddy to be held without bail, the feds shipped him outta the Lower Manhattan federal courthouse and off to Metropolitan Detention Center in Brooklyn ... a facility notorious for its brutal, sometimes fatal, conditions.

MDC Brooklyn's been the temporary home to several celeb inmates in recent years, including R. Kelly, Fetty Wap, Michael Cohen, Allison Mack and Ghislaine Maxwell. Now, Diddy will be settling into his new surroundings -- a horrifically stark difference from his Miami and L.A. mansion lifestyle.

Federal judges have called out the Bureau of Prisons for alleged inhumane conditions at the facility, and back in June, an inmate was murdered there. And, then there's the food.

MDC's meal plan has dinner options slated with meals like turkey roast or tofu with rice, boiled potatoes, beets and whole wheat bread with a beverage ... which sure as hell ain't a fine Italian wine.

It's a far cry from Diddy's normal diet, which we hear typically includes things like duck, steak, organic veggies, shrimp, salmon, crab and cheese platters.