Lindsay Lohan In Nightclub Fight

The witness tells us ... the incident began after a major misunderstanding between the two sides while they were partying at the Smoke and Mirrors nightclub inside the hotel.

We're told the girl was hanging out at the booth next to Lindsay's ... when Lohan tapped her on the shoulder aggressively and asked, "Did you bump into me?"

The girl denied the bump ... and suggested the real culprit could have been one of the guys in Lindsay's group. She then pointed to one of the men.

Lindsay replied, "That's my dad ... why would he bump me?"

The girl shot back, "You go clubbing with your dad?"

That's when Lindsay snapped ... so says the witness ... who claims Lindsay began cussing at the girl and then threw a drink at her face.

We're told Lindsay's dad realized what was happening .... grabbed his daughter ... and bailed.

Michael Lohan appeared on "TMZ Live" Wed and adamantly denied his daughter chucked any beverage that night ... claiming it was Lindsay who was the real victim and the recipient of the liquid projectile.

Either way, the other girl is currently weighing her legal options.