A man had three fingers cut off and his left arm almost severed by a meat cleaver after rowing with two men about their dogs not being on leads.
The victim, 39, had remonstrated with the men for failing to control their animals after they started biting him.
The man was followed by the pair, who goaded the dogs into attacking him before slashing him with the cleaver.
The attack happened in Shepherds Bush, west London, on Tuesday. Police said his injuries were "horrific".
The victim had been walking in Jeddo Road at about 2030 BST when the animals, a Great Dane and a Staffordshire Terrier, started biting his ankles.
When he shouted at the men to put them on leads, he was followed into Kinnear Road, where the dogs were set on him.
One of the men produced a meat cleaver and slashed at him, causing deep head wounds.
Three of the victim's fingers on his left hand were severed and one little finger later had to be amputated.
His left arm was almost severed at the elbow and there were several cleaver wounds to the backs of his legs leaving him with severe tendon damage to his legs and elbow.
The man managed to get into the garden of a house and raise the alarm by tapping on a window.
His attackers, described as black, about 5ft 7in tall, and in their mid- to late-twenties, fled the scene.
Det Con John Stewart said: "This was a truly deplorable attack on the victim which was completely uncalled for.
"He is very lucky to be alive and although his injuries are horrific I fear they could have been a lot worse."