Harley Pasternak: Paparazzi Arrested For Threatening to Kill Celebrity Trainer

A paparazzo was arrested for allegedly threatening the life of a huge celebrity trainer ... and then ominously reaching for what the trainer believed was a deadly weapon.

Harley Pasternak -- who trains Kanye West, Katy Perry, Miley Cyrus, Usher, Rihanna and a slew of other celebs -- told cops paparazzi showed up at his home Sunday because they got a tip he was about to train Robert Pattinson.

Harley says he told the paps Pattinson wasn't coming to his house but they didn't believe him. The paps got more aggressive and came onto his property. Harley told police he then walked out and ordered the photogs to leave, and that's when the photog in question threatened, "I'm gonna kill you and your family."

Harley says the photog reached for something in his car ... fearing it was a weapon, the trainer subdued the guy.

The photog denied making any death threats and insists Harley was the aggressor ... attempting to pull off his hubcaps with his bare hands. But a neighbor told cops she heard the death threat loud and clear. That was enough for police to hook the guy up and haul him to jail.

By the way ... the photog admits he was reaching for a weapon of sorts ... pepper spray.