Paris Hilton's boyfriend River Viiperi has been arrested for misdemeanour battery.

The 31-year-old socialite and her 21-year-old beau were partying at Las Vegas' XS nightclub inside the Encore hotel in the early hours of Monday morning (01.10.12) when the Spanish model punched an onlooker in the face.

According to gossip website TMZ, a male party-goer filed a police report claiming his girlfriend was ''making out'' with Paris on the dancefloor when a jealous River punched him so hard that he needed medical attention.

The leggy blonde - who was formerly banned from all Steve Wynn owned hotels after being arrested for cocaine possession - is adamant she wasn't kissing another girl and that River simply stepped in to protect her from a couple who were harassing her.

An insider told RadarOnline: ''Paris is steadfast that she never made out with another girl at the nightclub in Las Vegas. Paris is very upset that River got arrested because it was his first trip to Las Vegas.

''According to Paris, another couple had been harassing them at XS, and the guy was getting to close to the hotel heiress, and River just shoved him away from his girlfriend. River didn't hit the dude, he was just protecting Paris and trying to give them a little space.''

River was released after the police arrived on the scene, but the victim told hotel security he plans to press charges.

The new couple had been enjoying a ''fun and adventurous'' romantic mini-break in Hawaii before the altercation took place.