9:40 AM PT -- Sources close to Nipsey tell us the men he was fighting first attacked him because they thought he was someone else. We're told the men accused Nipsey of starting a fight earlier in the night inside the club, but Nipsey had nothing to do with it. The whole thing was started as a result of mistaken identity and Nipsey was only defending himself.

Nipsey Hussle was smack in the middle of a crazy clash where punches were thrown and tasers were pulled ... all on the heels of celebrating Nipsey's GF's birthday.

The intense melee went down late Sunday night in the parking lot of Nightingale in WeHo. You can see the Grammy-nominated rapper and his crew throw haymakers with a couple of guys that had confronted them earlier that evening.

We got the rapper and his GF, Lauren London, arriving at the club. O.T. Genasis was also there and it was shortly after that when our camera catches the first face-off ... it's at the very end of the clip. Less than an hour later, the brawl started.

You can see Nipsey in the middle of it all ... throwing several punches as valet workers scurry to get away from the chaos. Nipsey eventually cools down but things quickly escalate again. Moments later, someone -- presumably security -- pulls out a taser and things finally de-escalate.

Law enforcement sources tell TMZ ... cops showed up to the scene but everyone involved had already left. No arrests were made.