Chris Brown Fight Witness Says CB Swung First

after Frank disrespected the singer in front of his boys ... an eyewitness says. According to someone who was on scene, Chris lost his cool in the parking lot of Westlake Recording Studios in L.A. Sunday when Frank refused to shake his hand. Frank simply laughed at the gesture ... and Chris was so ticked that he cold clocked Ocean and started an all-out war. 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.