The 46-year-old man suffered severe injuries after the dog dragged him around Cherry Street in Blackburn by his hand.
The man has undergone surgery at the Royal Preston Hospital.
Lancashire Police said they had established that the dog was not illegal under the Dangerous Dogs Act, but was a "pit bull-type breed".
'Very aggressive'
Officers received several calls from residents about the attack, which happened late on Saturday night.
When police teams arrived the man had managed to free himself, but the dog was still loose.
It was described as "very aggressive" by officers and was shot on a nearby grass banking.
A police spokeswoman said inquiries into the attack were continuing.
The Dangerous Dogs Act 1991 bans the breeding and sale or exchange of four kinds of dog - pit bull terriers, Japanese Tosas, the Dogo Argentinos, and the Fila Brasileiros.
Cross-breeds of the dogs are also covered by the law.
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