A six-year-old girl is in hospital after part of her ear was bitten off by a dog.
The child also suffered injuries to her neck and shoulder when she was mauled by the animal in a park in Chingford, Essex.
Police said the unprovoked incident happened at about noon on Saturday at Pole Hill, Woodberry Way, in front of the girl's parents and her younger brother.
She was taken to a hospital in Essex for surgery, where she is said to be in a "comfortable" condition.
Officers and paramedics arrived at the scene soon after the attack, but the dog and the man with it had gone, the Metropolitan Police said.
A spokesman said "all possible steps" were being taken to trace them.
The dog is thought to be a black-and-white bull terrier. The man is described as white, in his late 30s or early 40s, with cropped blond hair.
Detective Sergeant Stuart Cheek, of Waltham Forest police, said the "appalling incident" had left the girl with serious injuries that could scar her for life.
He added: "I would appeal to anyone who saw what happened, was in the area at the time of the attack, or noticed anything since that might assist the investigation, to get in touch as soon as possible."
Anyone with information should call police on 020 3276 0967 or Crimestoppers anonymously on 0800 555 111.