R. Kelly's Chicago Recording Studio and Alleged 'Sex Cult' Den Visited by Cops

Cops paid a visit to R. Kelly's recording studio in Chicago, which has been featured on Lifetime's docuseries as an alleged "sex cult" base where the singer held girls captive.

Law enforcement sources tell TMZ ... officers dropped by Kelly's westside music studio Tuesday evening, but it appeared to be vacant.

We're told there was no call to police that triggered the visit ... cops went to check out the spot because it's gotten so much publicity from "Surviving R. Kelly." The docuseries portrayed the location as a hub for Kelly's alleged sexual abuse of women.

Interestingly, there was reportedly a party there Tuesday night, on what was Kelly's 52nd birthday. Our sources say cops had no knowledge of the event. We're told officers will continue to patrol the area.

On 'Surviving' ... the parents of one of Kelly's alleged victims, Azriel Clary, go to the R&B singer's studio and throw rocks at it, trying to get her to come outside. The studio is currently listed for sale.

As we reported ... Chicago prosecutors are urging R. Kelly victims to come forward, and we're told at least one victim and the families of 2 others are doing just that ... and believe they have sufficient evidence to prosecute him.