The massive mastiff-cross pounced on the spaniel while out for an afternoon walk in Fettes Row on Wednesday.
Heroic passers-by battled for five minutes to haul the dogs apart while a vet from a nearby practice rushed to help but the spaniel died from its wounds.
“When you see a dog carry another dog off like that the strength is unbelievable,” said lorry driver Andrew Cameron, 59, who stopped to help.
“I’m a small dog owner myself which is part of the reason I jumped in - I wouldn’t want it to happen to me. I’d never drive by that without intervening.”
The owner of the spaniel fell and smashed his head on the pavement during the struggle - needing hospital treatment for a minor injury.
Police were called and confirmed that a man, believed to be the owner of the mastiff-cross, was arrested and charged over the attack.
Mr Cameron, from Livingston, had just made a delivery in Cumberland Street and was at the corner of Fettes Row and Dundas Street shortly after 2.15pm.
“I didn’t actually see what happened at first but I heard the spaniel screaming and saw its owner getting back up off the pavement,” he said.
Mr Cameron stopped his truck in the middle of the street and leapt out to join other passers-by in trying to stop the attack with their bare hands.
“The owner of the big dog was pulling his dog around the neck trying to stop it dragging the spaniel up the street.
“Others came over too in a vain effort to get the dog off but we were too late.”
Good samaritans tried to prise the mastiff-cross away after it clamped its jaws around its victim’s head.
Other passers-by tried to keep another spaniel, also belonging to dead dog’s owner, away from the attack in case it too was mauled.
A receptionist from the nearby Dundas Veterinary Group was passing and rushed back to raise the alarm with colleagues.
“One of our veterinary surgeons and a nurse did attend a distressing incident yesterday afternoon involving a dog attack on another dog,” said a spokeswoman for Dundas Veterinary Group.
“Sadly one of the dogs passed away despite our best efforts, which is very sad. We’ve expressed our deepest condolences to the owner.”
A Police Scotland spokeswoman confirmed officers were called to Fettes Row at around 2.20pm on Wednesday.
“A man was arrested and charged after a spaniel was attacked by a mastiff-cross in the Fettes Row area,” she added.
“Despite the efforts of members of the public and a local vets, the Spaniel sadly died. It’s owner was also taken to hospital as a precaution having fallen during this incident.
“The Mastiff-cross has been surrendered to the authorities and officers are liaising with the Procurator Fiscal regarding this. Enquiries remain ongoing.”