Eminem is under fire over controversial lyrics in a freestyle he just released -- in which he threatens to punch Lana Del Rey in the face ... like Ray Rice.

Slim just released a new video featuring a freestyle in which he goes off on several topics -- before taking a shot at Del Rey.

Here's the verse in question ...

"I may fight for gay rights, especially if they d*ke is more of a knockout than Janay Rice."

"Play nice, bitch I'll punch Lana Del Rey in the face twice like Ray Rice in broad daylight in plain sight of elevator surveillance 'til the head is bangin' on the railin' ... then celebrate with the Ravens."