Officers were called to the High Street, Walton-on-Thames about 5.45pm following reports three men had argued inside the restaurant.
The argument continued outside and the victim, a man in his fifties, lost consciousness after allegedly being assaulted.
The two other men, aged in their twenties, fled the scene and were arrested on suspicion of grievous bodily harm at a property in Walton. Both remain in custody.
The victim was taken to a hospital with life-threatening head injures and his next of kin have been informed of his condition, police said.
Detective Sergeant Antony Preston is leading the investigation and called for any witnesses to come forward.
He said: “The restaurant and surrounding streets would have been extremely busy, and it is imperative that we speak to everybody who was in KFC at the time, or who saw or heard the commotion outside.
“I am also keen to speak to anyone who was driving past the scene, and any delivery drivers collecting orders from KFC, as it is likely that their dashcam or helmet cam may have captured what happened.”
The public in Walton-on-Thames may notice an increased police presence as a result of the investigation, Mr Preston added.
Anyone with information should call police on 101 and quote reference PR/45200091641, or alternatively do so anonymously by calling Crime Stoppers on 0800 555 111.
With reporting by PA Media