The owner, Daisy Jenkins, cradled her dying American bulldog after it was stabbed repeatedly by Wayne Stearman, who said he carried out the attack to try to stop it killing his terrier.
One of the blows was so forceful that the knife was said to have bent in half in what one witness described as a "bloodbath".
Mr Stearman was arrested over the incident, in which he used three knives, and held for 22 hours but has since been released without charge.
The attack took place after the bulldog, Tyson, was said to have attacked his dog, Monty, in Hampstead, north London.
Miss Jenkins, 41, begged passers-by to help her as Tyson died on the Aspern estate. "We were just clinging together right up against the fence and trying to keep away from the knives," she said.
"I could feel bleeding but I just held him tightly. I just kept saying to Tyson, 'Hang on, pleas hang on, pull through.'
Allison Havey , who witnessed the attack, said: "It was a bloodbath. She was covered in blood and she was just saying, 'Please help. Help me make my dog live.' It was very sad."
A vet was called but could not save Tyson.
Mr Stearman, 41, described the bulldog as a "lunatic" and claimed it had been the subject of "hundreds" of complaints.
"I'd just stepped out my house and the dog went for Monty," he said. "It wasn't on a lead. It locked its jaw on to Monty's head. It was like a lion on top of a gazelle. I was screaming for help and the owner was nowhere to be seen.
"When she did finally turn up five minutes later all she did was stroke her dog. I was kicking and punching it but it wouldn't budge so I went into the kitchen and got three knives.
"I'm not proud of what I've done, but what was I supposed to do? Stand there and let it kill my dog?"
Mr Stearman could be charged with criminal damage but was not expecting any further action to be taken against him. He claimed that the bulldog had tried to attack his dog three months previously.