The victim was punched to the ground and repeatedly kicked after being set upon in Victoria Park, off Ferry Road at 9:30pm on Sunday.
The attack only ended when a woman walking her dog came across the incident and shouted at the youths until they fled.
Local residents today said there has been growing concern about safety in the park, which has been plagued with gang fights, under-age drinking and vandalism.
Their fears prompted the city council to agree to new lighting being installed in a bid to deter groups of youths from gathering there, particularly during the summer months.
The victim of Sunday's incident was taken to Edinburgh Royal Infirmary for treatment to cuts and bruises to his face.
He had been walking through the park when the gang, which included one teenage girl, started talking to him as he passed. Suddenly, a number of the youths began assaulting him, leaving him lying on the ground as they kicked him.
Police are understood to be checking CCTV from the area in a bid to identify the suspects.
David Pestell, a 46-year-old surveyor who lives near the park, said: "Problems in the park usually kick off in the lead-up to summer, with groups of youths drinking there.
"In recent weeks, I heard there had been a knife fight involving gangs there. I think the park has attracted gangs from the Fort housing estate and Leith who meet gangs from places to the west like Wardie.
"Residents are concerned about walking in the park, but that comes as much from a fear of crime than the number of incidents. The incident on Sunday is the kind of situation which increases that fear."
Stewart Fowlie, a member of Trinity community council, added: "I'm sure police and residents will both be horrified because it has been quieter recently.
"There's been groups of youths, most of whom don't seem to be from the Trinity area, gathering there. That's led to vandalism, such as stones being thrown at windows."
Last September, a children's play area at the Victoria Park was attacked by fireraisers for the second time in six months. A climbing frame, a wooden bridge and other play structures were set alight.
A city council spokeswoman said: "We are in the process of installing additional new lighting in Victoria Park. The first phase of this will be completed shortly and the second phase will be considered following consultation later in the year."
A police spokesman confirmed they were investigating Sunday's attack, and said: "One of the suspects is described as male, white, 17 years old, 5ft 9in tall with a slim build, short dark hair in a side parting and wearing sunglasses. Another suspect was known to be wearing a black top with white flashes on the sleeve and black trousers."
Anyone with information is asked to contact the police.