EXCLUSIVE

Nick Diaz ended up in the middle of a huge brawl while partying at Hyde nightclub in Vegas early Wednesday morning ... TMZ Sports has learned.

A rep for the club tells us the UFC fighter went to a bathroom outside the club around 2:30 AM, and accidentally stumbled into another customer ... who started talking smack.

We're told Nick apologized and said he didn't want any trouble, but 3 other men then joined the first guy and jumped Nick ... who tells us, "I didn't start it. They didn't know who I was."

He tried to defend himself, and got hit hard in the head -- and then the fight spilled out of the bathroom, and out onto the Bellagio's casino floor.

The melee was insane -- the club's rep says the attackers were swinging chairs and brawling with club and casino staffers. They were eventually removed by security.

Nick told the Bellagio he didn't want to press charges, and told us, "They got the worst of it."