The eight-year-old is in a serious condition in hospital after being attacked in a block of flats.
A boy is being treated in hospital after being bitten in the head by a dog, believed to be an XL bully.
The eight-year-old was attacked in a communal area of flats near Wadham Road, Bootle at about 17:20 GMT on Saturday, Merseyside Police said.
He was taken to hospital where he is in a stable but serious condition.
A 49-year-old woman and a 30-year-old man, who are not related to the child, have been arrested, while the dog has been seized by police.
The pair were detained on suspicion of being a person in charge of a dog dangerously out of control and causing injury.
They have been taken into police custody for interviews.
Image source, Google
Officers said they had spoken to witnesses and appealed for anyone with information to contact them.
Det Insp Gary Stratton, from Merseyside Police, said: “I want to reassure people that [the boy] is receiving the best possible care and treatment for his injuries, which are described as life-changing.”
New rules for owning American XL bully dogs have recently come into force in England and Wales, and will be implemented in Scotland later this year.
It is illegal to own one without an exemption certificate, and registered dogs must be kept on a lead and muzzled in public.
Det Insp Stratton said: “We have seized the dog and extensive efforts are underway to establish exactly what happened.
“This case highlights in the starkest terms the potential dangers of dogs, and I would appeal to anyone with information about dangerous dogs in their area to contact us so that we can take pro-active action.”
