Luigi Mangione says the cops who arrested him at a Pennsylvania McDonald's got a DNA sample with a little trickery ... by offering him a snack.

The accused murderer's legal team is now arguing police violated Luigi's rights when they arrested and questioned him ... and he wants the evidence they collected thrown out ... according to new court documents.

In the docs, obtained by TMZ, Luigi says cops pulled a fast one on him after arresting him at McDonald's in Altoona, PA ...  giving him some food and soda at the police station as a means of getting a sample of his DNA.

Luigi's lawyer says the DNA samples are "poisonous fruits" obtained from an illegal search.

What's more, Luigi claims the cops who first approached him at the Golden Arches did not have proper legal justification to detain him ... and he says the officers illegally seized his bag and removed a notebook with his so-called "manifesto" as a way to sway public opinion against him.

Luigi's legal team is asking the court to exclude the evidence police obtained during their search. The documents were filed in his state case in Pennsylvania, where he's charged with possession of an unlicensed firearm, forgery, and providing false identification to police.

In New York, Mangione has pleaded not guilty to 11 state charges ... including first-degree murder, second-degree murder as crime of terrorism and multiple weapons charges. He faces federal charges of murder through use of a firearm, two stalking charges, and a firearms offense.