A Scotland Yard firearms squad was called in to shoot a dog dead after it mauled five police officers during a raid.
One Pc is facing skin grafts on his body after the "pit bull-type" animal went beserk as its owner was arrested on Thursday.
All five constables were taken to hospital with leg and hand wounds as chaos ensued during a morning swoop in Newham, east London.
A dog unit and CO19 Specialist Firearms Command were called to "contain" and shoot dead the animal while bloodied officers arrested the suspect, in his 20s, for grievous bodily harm and kidnapping.
Blood stained scene of the mauling
Four of the officers were said to be in a serious but stable condition. The remaining Pc suffered minor injuries.
Police said the raid was part of Operation Big Wing, a major Scotland Yard purge on wanted suspects across the capital.
Jenny Jones, the Green Party candidate for London mayor, said the dog bite raid appeared to be "an expensive mistake".
She said: "Calling in CO19 just seems such an expensive way to deal with a mad dog. Could they not have used a Taser?
"Police should have done their homework before carrying out that operation. They should have realised the potential for a dangerous dog at the premises."
Pools of blood were visible near the entrance to the property in Albert Square, the Press Association reported.
This is the house the police arrived at with an arrest warrant
A force statement said: "Police attended an address in Albert Square to execute an arrest warrant at approximately 9am... While officers from Newham attempted to arrest a male suspect, they were attacked by a pit bull-type dog.
"A dog unit was called to the scene and CO19 Specialist Firearms Command also attended. The dog was contained and subsequently shot dead."
Dog bite incidents in the UK have risen 79% in London and 43% nationally in recent years, according to figures obtained by the Kennel Club.
The operation was part of a 48-hour operation which saw hundreds of officers carrying out searches for people wanted by police or who had failed to appear at court.
A total of 1,619 "visits" were made across London, the force said.
Neighbours tried to help the police as the animal clamped its jaw around one officer's leg.
The dog was thought to resemble a Staffordshire bull terrier.
Commander Stephen Watson said the officers came under "sustained attack".
"All five were hospitalised. Of the five, two might be described as walking wounded whereas three sustained serious injuries which will necessitate further medical intervention," he said.
"I think what our officers have confronted is the bravery that they very often display in protecting Londoners on a daily basis.
The Newham street where the officers were attacked
"One man is in custody, inquiries continue, but the person was arrested in line with the original purposes of the inquiries and subsequently with offences concerning the Dangerous Dogs Act."
One officer was left with blood pouring from his hand, while another was forced to jump on the bonnet of a car in an attempt to kick the dog away, locals said.
Amid reports that the animal had frightened neighbours in the past, campaigners said the incident illustrated the need to strengthen the law against dangerous dogs.
Dave Joyce, of the Communication Workers Union, said: "If these police officers were attacked on private property they could find themselves with no protection under the current law, as many postal workers do.
"This has gone on far too long. Our campaign has gained the support of both the Scottish Government and the Northern Ireland Government who changed the law in 2011 - the English Government must stop dragging its feet and act fast."
David Urpeth, an expert in dog attack cases at law firm Irwin Mitchell, added: "This case demonstrates the enormous damage that can be done in a dog attack, which can affect people's lives forever.
"We are eagerly awaiting an announcement from the Government regarding potential changes to the legislation regarding dangerous dogs and reports of this kind highlight why it simply can't come soon enough."
A neighbour described how one officer leapt on to the bonnet of a car to escape the dog.
David Clarke said he had previously warned Newham Council that the pet was dangerous, after he reported it for attacking another dog.
The 72-year-old said: "That was a dangerous dog. I informed the council about it, which is why the fencing was put up.
"It has already attacked another dog. The owner never had it on a muzzle or a lead, but he did after the previous attack. He didn't take it out a lot.
"I think people shouldn't have those dogs. They should be completely banned."
He said of the attack on the officers: "The dog was holding on to his leg.
"He managed to get to the wall but the dog got over the wall and was trying to attack him again.
"He scrambled on to the car, stayed there for a minute. He got down when another officer was screaming and yelling."