She Screamed 'This is F***ing America!!!'

An eyewitness to the Britney Spears slap at the hands of security for NBA phenom Victor Wembanyama tells TMZ ... he clearly struck her and she was furious.

Brian Grajales, a Canadian who was outside Catch restaurant Wednesday night says he saw Britney approach Victor's entourage, clearly wanting to take a photo with him.

I WITNESSED THE SLAP

Brian says she "infiltrated" her way into his circle, leaned in, and in a British accent, said, "Excuse me, sir ... excuse me, sir." She then touched Victor's back, and that's when he says the head of Victor's security backhanded her right in the face.

Britney Spears Slapped by NBA's Victor Wembanyama Security, New Photos

One law enforcement source told TMZ,…