Dog-lover Samaia King said sister-in-law Emily and a friend were walking Mozart and another dog when the large dog escaped from a property in the street, and began attacking Mozart.
Emily was bitten as she intervened and her friend dragged their dogs to safety.
Council animal control officers were called and impounded the offending dog at an Exeter Crescent address. Emily returned home later in the afternoon with medication to prevent infection to a bite on her leg, but Mozart remained at the vet with bites and tears about the stomach area and was expected to have surgery, King said.
Animal control confirmed it had seized a large dog thought to be "a type of mastiff", and an investigation was underway with staff contacting the owners.
Police national media centre staff said it did not appear police had been involved.
King said the dog appeared to have got out of its property and "probably" acted in protection of its territory.
