Having been unaware of Gillete Stadium being a restricted area, I attempted to fly in the vicinity of stadium very early this morning figuring I could capture some nice photos/videos from 500ft up without doing a direct flyover. Within a few moments after takeoff, I received a warning of this "no fly zone", I hadn't cached a map before flying however, it was very apparent by the red circle now showing in the app. Although I was still a distance from entering the so called "red zone" I noticed my P3P descending and was unable to take back control with sticks. I eventually turned around my Phantom with it now pointing away from "no fly" zone and was able to safely bring it back.