Police are investigating an alleged dog attack that took place on Wednesday evening in Spinola and which ended in a cat being seriously injured.

The incident took place at around 7.15pm, with the police having to be called in after bystanders tried to free the cat from the dog's mouth, witnesses said. 

"It was horrid. I ran to help the cat and after finally opening the dog's jaw, it was covered in blood," said one bystander, who will also be giving the police a statement on Thursday. 

"The cat is apparently alive but it has lost two legs," the bystander said. 

The police could not provide any addition details on the attack but said investigations are ongoing.

Other bystanders said the dog's owner was present at the time. The cat was, for some reason, thrown into the sea and had to be rescued by three people who dove in after it, the bystander said. 

Another person who was on site when the incident alleged there had seen similar attacks in the past.