A man was rushed to hospital on Friday evening after being attacked by a dog, believed to be an American bully XL, in a south London park.
A spokesperson for the Metropolitan Police said the victim, aged in his 40s, was bitten on the arm in Pasley Park, Walworth, just after 18:00 BST.
They said that the owner fled the scene with the grey dog before officers arrived.
Efforts are being made to track down the owner, the spokesperson added.
They told the BBC: "There have been no arrests. Inquiries are ongoing."
Lawyer and writer Ness Lyons said the XL Bully "jumped a fence" and attacked the man at her local park.
Posting on X, formerly Twitter, Ms Lyons wrote: "Man was bitten badly in several places including his abdomen.
"Police and ambulance came, but it took an hour. Horrifying.
"The owner grabbed his dog and legged it."
'Urgent work'
Some residents in Walworth near to Pasley Park said they "lived in fear" of the American XL bully dogs. Others felt the breed should be "muzzled".
One man, who didn't want to give us name, told BBC London: "I tend to walk on the other side of the street when I see one and it does worry me for children and other vulnerable people."
But another man in the park said it was an issue with the owners rather than the dogs.
A number of recent attacks in England - including one near Walsall in which a man died - prompted the Prime Minister Rishi Sunak to say he had ordered "urgent work" to define and ban the animals.
Another recent dog attack on an 11-year-old girl in Birmingham provoked the debate about banning certain dog breeds.
However, the Dog Control Coalition, a group including RSPCA, Battersea Dogs Home and the Royal Kennel Club, said banning specific breeds was not the solution - pointing to "irresponsible breeding, rearing and ownership".
BBC Verify reported that 10 people died because of dog bite injuries in England and Wales last year.
Last year there were nearly 22,000 cases of out-of-control dogs causing injury. In 2018 there were just over 16,000, a BBC investigation found.
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