A guide dog was repeatedly kicked as his blind owner stood helplessly by.
Will McKinlay, 41, had let Nash, a golden retriever cross, run on Hove seafront.
Families watched in horror as a man, having a barbecue, threw three-year-old Nash to the ground and kicked him.
The attacker, a man in his mid-40s, only released Nash when a friend with Mr McKinlay ran over and pulled him clear.
Mr McKinlay said: "I've never experienced anything like this and I've had guide dogs for 20 years.
"I'm very upset. Nash is a wonderful dog and is well-behaved. He is very traumatised by the event."
Mr McKinlay and his friend Lewis Herriott, who is sighted, had let Nash off the lead for exercise.
Mr Herriott, 40, said: "It was almost like a bout of road rage.
"I've never seen anyone behave like that. Other people were shocked."
Mr Herriott, a Brighton dental technician, said Nash had to be given painkillers for bruised ribs.
"Nash is a big softie and he has been very quiet since the attack. We don't know whether it will affect him as a working dog."
A police spokeswoman said: "We can understand Mr McKinlay was very upset at what happened and we will speak to him and take relevant action.
"But dog owners do have a responsibility to keep their dogs under control."
The two men, who live in St Aubyns, Hove, are appealing to anyone who saw the incident to contact police on 0845 6070999.
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