​Police were confronted with a nightmarish scene Sunday at San Francisco's Immaculate Conception Church, as a naked, blood-soaked man was arrested after transforming the Bernal Heights chapel into an abattoir and bellowing how he was forced to kill his pet dog as it was demon-possessed.

SF Weekly first reported on this disturbing story yesterday; bizarre details released today by the San Francisco police department unnerved even veteran officers.

At 5:23 p.m. on Sunday, police received a frantic call from an eyewitness who claimed that a 6-foot-tall, shirtless male had pulled up in front of Immaculate Conception Church in a yellow pickup truck and proceeded to begin hacking his pit bull with a hatchet. The caller then informed police that the man entered the church while carrying the mortally wounded animal.

Officers from the Ingleside Station arrived at the church and saw fresh blood pooled on the sidewalk. Literally following a trail of blood, they entered the church. On a table within was a scene out of a horror movie: A blood-stained hatchet alongside a pair of pants and shoes soaked in blood.

The bloody trail led police through another door into a courtyard, where they discovered 41-year-old Joseph Byrnes, naked and covered in blood. He was crouching over a dog Inspector Ricardo Galande described as "chopped to pieces," oozing blood and brains out of its many cuts.

Ordered to stand and put his hands in the air, Byrnes told officers, "I had to kill my dog, he had the devil in him"  He later told police "The devil was in my dog. I had to get the devil out."

The dog, amazingly, survived the night but was euthanized on Monday morning.

Byrnes, who has no local address, was taken without further incident to San Francisco General Hospital, where he is currently being held in Ward 7L, the psychiatric wing. He is charged with burglary and animal cruelty.

The SPFD is currently uncertain if Byrnes has a criminal history. Questions about the slaughtered dog -- and its owner -- were not answered yesterday by San Francisco's Animal Care and Control, citing the fact that this is an ongoing investigation.