Police officers arriving at the scene on top of the hill seized the dog from the owner. The child was rushed to hospital for treatment where her injuries were declared not life-threatening, police said.
A witness said the “small brown pitbull-type dog” had been charging around the park when he heard frantic screams.
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Richard Woodburn, 53, told The Independent: “The dog had come up to me as I use the park as a cut-through to go home but I just ignored it. The owner was shouting at the dog to come back.
“Then I heard him shout louder and louder and then I heard screaming. I ran back up the hill but the dog was already being held down by the owner.
“The dad had to prise the dog off her head. He sustained injuries to his hands.”
He added: “At first I didn’t think anything was wrong because her mother was holding her so close. But the police wanted me to verify if there were any injuries.
“So she showed me the side of her head and it was matted with blood.”
The witness added: “The father was blaming himself for taking his kids to the park. How ridiculous to think like that? I really felt for him as he felt so guilty.
“The girl was crying to begin with and then was just in shock. Another older girl looked properly traumatised.
“When the police officer arrived to do first aid he told the paramedics that she’ll probably need stitches.”
A Met spokesman confirmed the dog was seized.
“The child was taken to hospital for treatment; their injuries are not believed to be life-threatening,” they added. “No arrests have been made and enquiries continue.”
The incident is the latest in a string of dog attacks - some of them fatal - to have taken place across the country in recent weeks.
The Cane Corso X is said to have shown no previous signs of aggression before biting the boy as he started to cry in his Barnsley home on Tuesday morning. The baby remains in hospital in a stable but serious condition.
Police said the attack was out of character for the dog, who has no previous history of aggression, with the owner describing him as being “pleasant” and “affectionate” with children.
A five-year-old boy suffered serious injuries after being attacked by a suspected Staffordshire Bull Terrier in Ashington on Sunday.
The child is currently being treated in hospital for serious but non-life threatening injuries, police said