Bec Davison, deputy director of the Brighton and Hove-based charity CRI, said the victim was well known to services in the city.
She said: “We have worked with him for a long time – we had a good relationship and he seemed quite happy.
“It's extremely distressing for everyone here. The saddest thing is that it's quite easy for someone who's a long time rough sleeper to become invisible.
“But this man was vulnerable and he had a real story.”
The charity workers who found the body were volunteering for Project Antifreeze, a Brighton and Hove-based homeless project.
Julian Haddow, the project's leader, said the three volunteers had been traumatised by their discovery but that they would be given “full support”.
Detective Chief Inspector Ian Pollard, who is the leading the investigation, said “no stone would be left unturned” in the hunt for the man's killer.
He said: “"This was a vicious attack, the motive of which is not yet known nor how the deceased's injuries were inflicted.
“"I know the street community are very close knit - but I would appeal to them to please get in touch if they have any information that could help us with our enquiries no matter how relevant they think it is.
“This man was found next to a busy thoroughfare with cyclists, joggers and traffic. If anyone knows anything about what happened - please get in touch.”
Anyone who witnessed the attack or has any information that could assist police with their enquiries are asked to call 101 quoting Op Depot or alternatively leave an anonymous message with the independent charity Crimestoppers on 0800 555 111.
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