The Center for Science in the Public Interest in June said it was considering a lawsuit against McDonald's for giving away toys to lure children towards unhealthy meals.
The group said McDonald's had broken a pledged not to target children in advertisements that marketed calorie-heavy meals.
On Wednesday, Chief Executive Officer James Skinner said the advocacy group had it wrong, Crain's Business Journal reported.
"CSPI's twisted characterization of McDonald's as 'the stranger in the playground handing out candy to children' is an insult to every one of our franchisees and employees around the world," Skinner said in a statement.
"I want you to know we will vigorously defend our brand, our reputation, our food and our people. CSPI is wrong in its assertions and frivolous in its legal threats," Skinner said.