The 39-year-old was found in the back garden of a house near Hylands Park in Essex yesterday (August 19) after a party was held opposite the festival site.
Preliminary results from a post-mortem showed that the man died of head injuries.
An Essex Police spokeswoman said: "Detectives have launched a murder inquiry in Chelmsford.
"Police were called by the ambulance service just after midnight and are investigating the circumstances surrounding the suspicious death of a 39-year-old man who was found in the rear garden of a house on the A414 London Road, Chelmsford, opposite Hylands Park.
"His next of kin have been informed. Ten men have been arrested by police on suspicion of murder and they will be interviewed to ascertain their involvement in the man's death.
"Detectives from the Kent and Essex Serious Crime Directorate need to speak to everyone who attended what police believe was a private party at the property during the late hours of Saturday / early hours of this morning."
The arrested men, who are all thought to be aged between 23 and 35, remain in custody for questioning.
Meanwhile, police have named the 22-year-old who was found dead in a tent at the Staffordshire site as Timothy Brockhurst.