Photog Will Sue Over Lana Del Rey Video Shoot -- I Was Slashed By Thugs With A Razor

A Lana Del Rey film shoot turned into a bloody brawl ... according to a photog who claims he was attacked by 3 thugs with a box cutter ... to stop him from taking pics of LDR ... and he's gonna sue.

Karl Larsen -- a well-known concert photographer who shoots for Rolling Stone magazine -- claims in a lawsuit that will be filed later this week ... he happened on the L.A. River last June and saw Del Rey shooting her "Tropico" video ... and was confronted by 3 dudes from the production company.

Larsen says the men ordered him to stop shooting ... when Larsen insisted he was within his rights ... one guy threatened to smash his $14K camera gear and another tried to rip it from his hand.

Larsen claims one of the men whipped out a box cutter razor and slashed his finger -- which required stitches -- while another guy grabbed his camera and erased the memory card.

Larsen's suit against the production company, Black Hand Cinema, seeks $1 MIL minimum -- claiming the company has a habit of intimidating photographers.