Awful footage showed the moment a in a park as the police now search for the animal and its owner.
The shocking incident occurred while the boy was walking home through Queen's Park in Bolton on Monday afternoon.
The young boy was left with bloody wounds and a "hole" in his arm.
He received treatment in hospital, but he and his family have been left scarred by the attack.
The 14-year-old has a phobia of dogs and is "very shaken" after the incident.
His brother, Kashif Khan, that the owner of the dog eventually put the animal back on a lead and made off after the attack.
He said: "He's just a kid. They [owners] were in the park at the cafe. They had left the dog just next to it on the opposite side, but it didn't seem to be on a leash or tied to anything.
"My brother is scared of dogs. It started barking at him as he walked past, so he started running and then slipped. The dog ran over and started attacking him and wouldn't let go.
"The owner came and as soon as it let go of him, he ran away. The owners didn't help or call the ambulance or anything, they just ran off. They didn't even ask if he was okay.
"He went straight to hospital and my cousin called the police. He had a hole in his arm that was bleeding which the paramedics have plastered up. He's OK now but was very shaken up and shocked.
"Luckily it was just his arm and not his neck - it could've been so much more horrendous. It could have killed him. What else can you do in that situation? It's shocking. They just ran away like they didn't care.
"When we were kids we would go to the park regularly with no problems. But you have to be cautious, you don't expect a dog to not be on a lead."
Pictures showed the deep teeth marks on the boy's arm.
Greater Manchester Police is looking into the incident and enquiries are ongoing.
The incident was reported to the force at around 3.50pm on October 28. Officers took a statement from the victim when he was receiving treatment in hospital.
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