A gang of youths set their dog on a cat before throwing the feline over a fence.
Police are appealing for witnesses after the incident in Arne Close, Bewbush, Crawley on May 12 at 7.30pm.
The three boys were seen playing with a ginger cat and waving it in front of their white Staffordshire bull terrier.
The dog had tanned patches on its body and was off its lead and was repeatedly encouraged to attack the cat which the boys were holding.
However, when a neighbour spotted what they were doing the youths threw the cat over a garden fence.
The cat is being treated by a nearby vet for its injuries.
PC Gillian Sadler said: "This was a very nasty attack on the cat and has left both the animal and its owner traumatised.
"It is difficult to understand why anyone could behave with such cruelty to a pet.
"I want to speak to anyone who saw or heard the attack on the cat.
"There may be other people who saw the boys messing around with the cat before they set the dog on it or someone who recognises their descriptions.
"If you know anything at all please contact us."
The first boy is described as white and 14 or 15. He had light blonde hair that was shaved at both sides and was topless.
The second boy was also white and 14 or 15. He had dark brown hair and was wearing a dark coloured tracksuit with his hood up.
Anyone with information should call 101 quoting serial 1334 of 12/5, email firstname.lastname@example.org or call the Crimestoppers anonymously on 0800 555111.