A 12-year-old boy faces skin grafts and having a metal plate inserted in his arm after being savaged by a frenzied dog while playing football with friends in a field, police said today.

Sean Young was bitten 23 times by the Staffordshire Bull Terrier and was only saved from further injury by passing workmen who pulled the dog off him and pinned it to the ground for 20 minutes.

The dog's owners, who called a vet after the attack, immediately asked for the dog to be put down on the spot.

Sean, of Boynston Grove, Sedgefield, County Durham, was playing football in the village of Coxhoe, with friends on Friday when the incident happened. The dog was being walked by a woman when it spotted the youngsters.

After apparently loping around the field chasing their ball it turned vicious after Sean did a handstand on the grass.

A Durham Police spokesman said today: ''At first it only chased after the ball a couple of times but when Sean performed a handstand it seemed to have the effect of upsetting the animal and sending it into a frenzy.

''The dog went for Sean and seized him by the arm, dragging him along the ground.

''Its bite was so strong it broke his right wrist and the Bull Terrier was only stopped when two workmen passing by in a van saw what was happening and leaped out to help.

''Sean suffered a total of 23 puncture wounds and is being treated at the University Hospital of North Durham where he is due to have a metal plate inserted into his arm and there's also the chance he could need skin grafts.''

The men who rescued Sean were field engineers Derek Hedley and Peter Campbell, both 36, who were heading back to their depot.

The police spokesman said that at first they thought the dog was playing with the youngster.

He added: ''They started to kick the dog and grabbed at its choke chain in an attempt to make it release its grip.

''They eventually forced the dog to let go and managed to keep the animal pinned to the ground until a local vet arrived at the request of the dog's owners.

''The couple, who had only had the terrier for a week, asked the vet to put it down, which he did on the spot.''

Sean's mother Denise Young, an administration worker with National Savings, said today: ''If it had not happened in a public place I dread to think what the outcome could have been.

''There's no way the boys on their own could have got the dog off and I'm grateful for what Derek and Peter did.

''Sean is still suffering flashbacks and understandably is very nervous about dogs at the moment but hopefully that will fade in time.''

Durham Police confirmed that the dog's owners have been reported for failing to keep a dog under control and a file is being prepared for the Crown Prosecution Service.