The sufferer, in 40s, was attacked in Pasley Park, Walworth, shortly after 6pm on Friday and was taken to hospital struggling accidents to his arm, the Metropolitan Police stated.
“The owner of the dog left the scene with the dog before officers arrived,” a Met spokesperson added.
“The dog is believed to have been a grey-coloured XL bully.”
Police are investigating and no arrests have been made.
Writer and lawyer Ness Lyons stated the assault occurred in her native park.
In a put up on X, previously referred to as Twitter, she stated: “Earlier this night an XL bully jumped a fence and attacked a person in my native park. Partially witnessed by my son.
“Man was bitten badly in a number of locations together with his stomach.
“Police and ambulance got here, however it took an hour. Horrifying.
“The owner grabbed his dog and legged it.”
Elsewhere, demonstrators rallied in central London on Saturday in opposition to the Prime Minister’s proposed American bully XL ban.
They clutched placards with messages equivalent to “don’t bully our bullies” and chanted “save our bullies”, whereas on individual was pictured carrying a T-shirt which stated “muzzle Rishi Sunak don’t bully our breed”.
Mr Sunak has promised to ban the breed underneath the Dangerous Dogs Act by the tip of the 12 months in response to a sequence of assaults, however homeowners of should not anticipated to face a cull of their pets.
On Monday, the Prime Minister’s official spokesman stated a “transition period” could be launched, with particulars more likely to comply with a session on the plan.
Owners may face a requirement to neuter their dogs and muzzle them in public, the Government’s chief vet has urged.
The Prime Minister’s official spokesman on Monday stated: “We might want to safely handle the present inhabitants of those dogs. Exactly what that appears like might be a subject for the session.
“And there will need to be some sort of transition period.”