Charlie Sheen -- Cops Called Over 4-Person Mansion Rager

Charlie Sheen doesn't know the definition of "low key" -- TMZ has learned, cops were called to his house last night ... over a party that was SUPPOSED to be a small, intimate gathering of friends.

Sources tell us, Charlie and 3 or 4 close friends were kicking it after he had spent all day shooting for "Anger Management." It was a long day and Charlie needed to unwind ... drinks, music, yadda yadda ... so he invited some pals over to relax.

But in typical Warlord fashion, we're told the intimate gathering quickly spiraled into an all-out rager ... and the neighbors finally called police to complain about the noise.

We're told cops responded to the scene ... and informed Sheen's security about the noise complaints. Charlie agreed to keep it down and, voila, the police left without further incident.