Prince Jackson -- My Grandma Was Mentally Abused

Michael Jackson's eldest child is putting his relatives on blast -- particularly Janet Jackson -- by posting what he claims is an angry group text message convo about his grandmother Katherine Jackson.

Katherine returned from Arizona late last night, but a few hours later Prince Jackson sent out a tweet that included the text message, dated July 23rd (Monday). In it Prince says, "This is enough so I am texting you for the simple fact that WE DEMAND TO SPEAK TO MY GRANDMA NOW!!!"

A reply from "Janet Jackson" reads ... "Don't let them pls." It appears the message continued, but Prince did not post the rest of it.

In his tweet Prince also says, "Although I am happy my grandma was returned, after speaking with her I realized how misguided and how badly she was lied to. I’m really angry and hurt."

He continues, "For this whole time, they denied us contact to our grandmother. If you continue with your lies I will continue with the truth.”

Prince added a reference to his father -- "As long as I can remember my dad had repeatedly warned me of certain people and their ways."