Police discovered the man being attacked by the Rottweiler-type dog in east London, on Sunday morning.
Officers managed to distract the animal away from the man before chasing it off, in New City Road, Newham. The dog was later caught and destroyed.
Armed officers were also called to protect the public because the dog was "dangerously out of control".
The victim was later taken to the Royal London Hospital, where he remains in a "critical condition".
The Metropolitan Police have begun an investigation into the incident, which happened at about 0700 GMT.
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