There are two types of completely broken legs in adult cats: open and closed. Open breaks are when the bone protrudes through the skin. There is a high chance of infection with this sort of break.
|Me and my broken leg - Photo by treehouse1977|
Leg breaks lead to shock and blood loss. There may be other injuries to internal organs. Don't assume the injury is confined to the leg. "Shock" in this instance means traumatic shock. Your cat may pant and have a rapid heart beat and high pulse. Try to calm your cat. Speak in a calming voice. Make sure your cat is as comfortable as possible when being transported to the vet for immediate treatment.
The leg has to be immobilized. This can be done with an empty toilet roll and tape for a break of the lower leg - below knee or elbow. For a higher break the leg is immobilized by binding the leg to the body. If unsure the best method and perhaps most practical is to place your cat in a towel and gently hold him or her while ensuring that the broken leg is in a natural position and static while held. The person holding the cat can be a passenger in the car that takes you to the veterinarian. It will no doubt be very difficult because the cat will want to move and he or she will need to be immobilized for a considerable time.