The opening of a new Marathon Burger — the restaurant chain founded by the late Nipsey Hussle — was marred when shots were fired at the event … TMZ has learned.

The incident took place in downtown Long Beach Sunday around 3:30 PM PT … and video is now circulating online which shows cops picking up a person and placing them in the back of a squad car.

Snoop Dogg and YG attended the event, but were gone long before the shooting began.

On Saturday, he, Nipsey’s former partner Lauren London and several of the late rapper’s close friends gathered together while an intersection in L.A. was renamed “Nipsey Hussle Square.”

Nipsey was shot and killed outside his Marthon Clothing Store on March 31, 2019.