Youths played football with a hedgehog before smashing it to death with a sledgehammer.
Police said the incident was "vile" and an investigation has been launched to identify the teenagers.
The attack happened at 5am yesterday in Rotherfield Crescent, Hollingbury, Brighton.
One witness said: "I was woken by the sound of lads playing what I thought was football. There were two or three of them and they were about 15 or 16. I heard one shouting 'goal'.
"Then I heard clumping noises and looked out of my window. One lad was using a sledgehammer.
"I thought they were smashing the pavement but then I heard one lad say 'Ugh - it's gone all over me'. He must have been splattered with blood."
The woman dialled 999 and saw the youths flee when a car passed by. She said: "I saw what was left of the hedgehog. It was flattened."
She described the attack as disgusting.
Another resident said: "I feed hedgehogs in my back garden with peanuts and dog meat. They're lovely.
"I can't believe anyone would do this."
Chief Inspector Kerry Cox, of Brighton police, said: "This is a serious matter and we will be investigating to identify who might have been responsible.
"We can't have young people going round doing things like this."
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