A dog falling from the roof of a two-storey building has struck a woman on the head, leaving her with a serious neck injury in Guangzhou City, Southern China.

Two women were walking in the city's Baiyun District earlier this month when one of them was suddenly hit by the dog.

Surveillance camera footage showed the woman slump unconscious after being hit by the animal, which witnesses said weighed at least 20 kilograms.

The accident could have been even worse — the other woman in the video is seen holding a baby.

Local witnesses described how they heard a loud sound and saw an animal falling to the ground.

"It was very loud, like a bag of cement falling down," said a local woman who witnessed the incident.

In a stroke of luck, the two women had been walking to a local clinic at the time.

A nearby doctor came to the injured woman's aid.

"I saw that her lips suddenly turned black … the doctor next door rubbed the area below her nose with some herbal oil," the witness said.

The woman took more than 12 minutes to regain consciousness.

"She had totally blacked out," said another local.

Dog flees the scene

Neighbours quickly called an ambulance and the unfortunate woman was sent to hospital, but the dog had already fled from the scene.

"The dog ran off at once. It was fine, because its fall [was] buffered by the person," said a witness.

As for the whereabouts of the canine— it has seemingly since stayed away from the scene of the crime, locals said.

"I often saw it (that dog) before, but these days, I haven't seen it any more since the accident," said the female witness.

The second floor of the building is used as a factory.

Workers said that they have never kept any dog there, and believe it may have entered the factory via another passage and jumped off the roof.

"It's such a rare thing. You've never heard of a dog jumping off a building, have you? Let alone a dog jumping off a building and hitting a person," said a worker from the factory.

The woman sustained fractures to the second, third and fourth bones of her cervical vertebrae and remains in hospital, doctors said.

Police are investigating the incident.