Bristol Palin Attacked By Heckler -- 'Your Mother's a Whore!'

Bristol Palin stood face to face with a very ANGRY Sarah Palin hater at the Saddle Ranch on the Sunset Strip last night ... after he screamed at her, "Your mother's a whore."

It all went down after Bristol rode the mechanical bull -- when the man shouted, "Did you ride Levi like that?"

During the confrontation, the man called Sarah "evil" -- and Bristol replied, "Is it because you're a homosexual and that's why you hate her?"

The man responded, "Pretty much ... and why'd you say I'm a homosexual?"

Bristol shot back -- "Because I can tell you are."

Bristol -- with her reality TV crew in tow -- eventually stormed out of the bar with her friends.