John Day had his faithful friend, Jill, a Lhasa Apso breed, for nearly 15 years when she was targeted by the "pit bull-type" dog on Hatfeild Street earlier this month.
Mr Day, 64, who uses a mobility scooter to get about, said: "We were walking along and she had no lead on, and when I got round the corner there was the other dog playing in the bushes. It wasn't on a lead either and had no muzzle but the owner was nearby.
"It just grabbed hold of my dog and that was it. It had hold of her for about two minutes. She's only a tiny dog, she didn't stand a chance.
"I'm disabled and just couldn't get there quick enough.
"The owner got hold of his dog and he walked away laughing and smiling. He was shouting at me 'pick your dog up'."
Mr Day, who has two other dogs, took Jill's body to the vets where he was told she had suffered severe internal injuries and a broken neck.
He has since paid to have her cremated and her ashes returned to him.
He says the horrifying incident has badly affected him, adding: "I can't sleep, I'm crying all the time. I've had her a long time. She would have been 15 next month.
"I've seen that dog owner a few times but not since then, but he can't hide forever."
Police have confirmed they are investigating the matter, having recorded a crime of a dog being dangerously out of control.
Enquiries are ongoing.