Chris Brown is at the center of an assault investigation after a woman claimed she was forcibly thrown off his tour bus in Colorado …TMZ has learned.

The incident went down Monday night outside Platinum 84 Gentleman's Club in Denver where Chris,were shooting a new music video. We're told a woman, who was not part of the production, claims she was forcibly thrown off Brown's bus after refusing to give up her cell phone. There were strict "no cell phone rules," because no one wanted video or photos secretly shot. The woman also claims her cell phone was broken in the melee.