The singer's attorney, Steven Greenberg, tells TMZ ... Kelly was moved Tuesday and claims the singer was only kept in solitary by prison officials as a form of punishment -- due to his celebrity and the prurient nature of his alleged crimes.

Greenberg tells us that although R. Kelly will be in gen pop -- which is normally considered more dangerous, especially for high-profile inmates -- he claims there are going to be security measures in place that should keep his client safe.

We're told R. Kelly will now be allowed 10 non-attorney visits instead of the previously allowed one. He'll also have a longer leash for phone calls and emails.

Perhaps most important -- for R. Kelly it seems -- is the fresh air he'll be able to experience now with other prisoners during rec time. He'll get to hit the yard if he wants.

Now, prosecutors confirmed R. Kelly was being moved to gen pop in a court filing of their own -- but added he was only in solitary briefly and at his request.

In the docs, obtained by TMZ, state prosecutors say R. Kelly had previously asked to be placed in a secluded wing of the prison called the Special Housing Unit with no cellmates, at first. They say he eventually did get cellmates.