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One of Mario 's fans may have taken "Let Me Love You" way too literally during a recent show ... reaching in and rubbing his crotch mid-performance -- and the R&B star clearly wasn't having it.

The incident reportedly went down Saturday during Mario’s tour stop in Detroit at the Masonic Jack White Theatre ... and in a now-viral clip, he's seen passionately serenading the crowd, inching toward the edge of the stage when a front-row fan suddenly slides their hand right onto his crotch.

The fan lingers for a moment -- fully cupping his private area -- until Mario shoos them off and appears to gently scold them. Gallery Launch Gallery Getty

According to the woman who recorded the video, she says Mario was actually "pretty nice about it."

To his credit, Mario d

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