Paris Hilton in alleged lesbian makeout session before boyfriend River Viiperi arrested

Paris Hilton made out with another girl at a nightclub in Vegas early Monday morning, and moments later, her new boyfriend was arrested for battery, at least that's what the alleged victim is telling police.

TMZ has learned a male clubgoer filed a report with the Las Vegas Metro Police Dept. claiming his girlfriend was "making out" with Paris inside XS nightclub at the Encore hotel when Paris' new boyfriend River Viiperi got upset.

The alleged victim claims Viiperi -- a 21-year-old male model -- punched him in the face sometime around 2:30 a.m. and injured him so badly, he needed medical attention.

The victim told hotel security he wanted to press charges against Viiperi, and when cops arrived to the scene, Viiperi was arrested for misdemeanor battery. He was cited and released without ever going to the police station.

Hilton had already left the scene by the time cops arrived, TMZ reports, but she has not been accused of any wrongdoing.

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Cops tell us the matter is currently under investigation.

Paris' rep did not immediately return TMZ's call for comment.