Chris Brown -- Accused of Punching Woman ... Vegas Cops Investigate

6:00 PM PST -- The LVMPD says when they went to the hotel to speak with Chris, they were not allowed in the room but later determined he wasn't there anymore. At this time they have not heard from the singer and their official report is for misdemeanor theft (for the phone) and battery.

3:00 PM PST -- A rep for the singer tells us the woman's claims are "undeniably untrue."

Chris Brown is at the center of a battery investigation after a woman claims the singer punched her in the face.

The alleged victim tells us she was at a private party in Brown's suite at the Palms Hotel in Vegas early Saturday morning. She says she was able to get her cellphone into the suite without his security noticing and when she took a pic of the singer, he went off, yelling, taking her phone and punching her once in her right eye.

The alleged victim, whose name is Liziane Gutierrez (we're only identifying at her request) says she left the party and called the cops a few hours later but was not taken to the hospital.

Law enforcement tells us they're currently investigating the incident and Chris is the suspect.

We've reached out to Chris' rep and lawyer ... so far, no word back.