Chris Brown threw the first punch in his brawl with Frank Ocean ... after Frank disrespected the singer in front of his boys ... an eyewitness tells TMZ.
According to someone who was on scene, Chris lost his cool in the parking lot of the famous Westlake Recording Studios in L.A. Sunday when Frank refused to shake his hand.
We're told Frank simply laughed at the gesture ... and Chris was so pissed he coldcocked Ocean and started an all-out war. As we first reported, Chris denies being the aggressor.
The fight quickly moved to the lobby of the studio ... and sources say the brawling posses demolished the place --"They managed to break almost everything in the room and scare all of the staff members working" says the witness.
We're told the receptionist called 911 ... and Chris booked it. As TMZ first reported, Frank stuck around for the cops ... and griped about how he felt "blindsided" by Chris.
One last thing -- we're told the parking situation at the recording studio is a serious hierarchy -- with the bigger artists taking the better spots ... and it's been that way for years. We're told Chris clearly felt he deserved the top spot ... and Frank disrespected him by taking it for himself.