A 13-year-old boy suffered a severe bite wound as he was attacked by a dog on a playing field in Medway.
The dog, thought to be a Staffordshire bull terrier, attacked Liam Monks, who had been playing football with friends.
Kent Police said the dog was with a group of five men who let it off the lead at the Garrison Sports Ground in Gillingham on Wednesday.
The teenager has had plastic surgery at the Queen Victoria Hospital in East Grinstead, West Sussex.
He said: "I thought I was going to lose my leg. I was lying on the floor and I looked around and saw a dog running at me at full speed.
"I went to stroke it and it took a big bit out of my leg and went straight into the muscle."
Police said Liam was sitting on the ground when the men went on to the playing field at about 1540 GMT and let the animal off the lead.
The dog went over to Liam and attacked him by biting him on the back of his leg, locking its jaws for about 15 seconds.
Officers said the men pulled the dog away and ran off towards the Great Lines War Memorial in Chatham.
Liam's mother, Debbie Monks, said: "I'm really annoyed that these guys just ran away.
"They're cowards and they need to be found and the dog needs to be found."
Pc Sally Haughton said: "The dog is described as being white with black spots all over it.
"It's thought that it may have been a Staffordshire bull terrier."
She said officers wanted to hear from anyone who was in the area on Wednesday and saw five men with a dog fitting the description.