Parents have spoken of their shock as hundreds of youngsters arrived at the MTV Crashes concert in Londonderry "out of their skulls on drink or drugs".
Out of 100 revellers taken to Altnagelvin Hospital's Emergency Department during the event, 30 were alcohol or drugs related.
And some of those who were taken into intensive care were aged under 16.
At one point, police used CS spray on two men who were fighting and couldn't be separated.
Security staff at the event turned hundreds of intoxicated young people away from the gates as they arrived in buses, while dozens more were treated at the medical triage area inside.
One mother who was there with her 13-year-old son and his friends said she was horrified by the chaos she witnessed.
Monica Reilly said: "Under-14s were supposed to be accompanied by a guardian, so I and a couple of my friends were there with our children.
"The issue wasn't really inside, but the problem was on the way in. I witnessed hundreds - and I mean hundreds - of young people out of their heads on drink or drugs at the gates trying to get in.
"They were getting off the bus and they were already off their heads. Young girls and boys and not a parent in sight.
"There were young girls who hadn't a clue where they were, who they were with or what they were doing. It was the saddest thing I have ever seen.
"Inside I saw other children taking tablets, God alone knows what they were, but by the effect they had on them they were illegal drugs of some sort. I didn't think I was easily shocked, but I never saw the like of that before.
"There is no way a child of mine will be back at this."
Stephen Kelly, a volunteer at the event, said buses were arriving packed with drunken children. He said: "Around 30 buses arrived at the event that the organisers knew nothing about and the doors of these buses opened, children spilled out on to the streets drunk out of their skulls.
"These young people were vomiting and staggering all over the place. We sent a bus from Ardoyne back full of intoxicated young people and wouldn't allow hundreds of others inside.
"Others were in such a bad state that we had to bring them inside the event to get them medical assistance. It was ridiculous how out of control so many of these young people were.
"I was asked by the organisers to help out and as a father of a 14-year-old, I was happy to do whatever I could to help. I was one of 200 stewards there on Saturday and there were 27 medics, including a doctor on site, as well as ambulance and police.
"Council safety wardens were also in attendance and no corners were cut by the organisers. But the problem was people were turning up at the event in no fit state and there's not much the organiser of an event can do about that. Having said that, there were 12,000 people there and 99% of them had the time of their lives, enjoyed the music and went home with no issue at all and it was the same for the Sunday night performance."
Despite the vast number of under-age drinkers in the area on Saturday, police made just one arrest. Paul Baylis, Emergency Medicine Consultant at Altnagelvin Hospital, said his department was stretched to a dangerous limit because of the wave of incoming patients from the MTV event.
He said: "There was a significant increase in attendances at Altnagelvin's Emergency Department from the early evening on Saturday through to the early hours of Sunday morning as a result of the outdoor MTV concert in the town.
"There was a significant influx who were aged around 13 or 14 up to their 20s and almost all of them showed evidence of drug use, either Ecstasy or amphetamines that produces a variety of problems. Some were very sick and needed to be moved on to our colleagues in Intensive Care.
"Out of the 100 overall attendances at the Emergency Department, 30 of them were alcohol or drugs related. There was also an increase in the number of cases of assault attending our service with the injured patients stating that they had been assaulted by drunk or stoned assailants.
"As a direct consequence of the MTV event our usual challenging Emergency Department Saturday evening environment became a more dangerous environment for all our patients, as our fixed staffing resources strived to cope with the additional workload."
Yesterday, police said CS spray was used during an altercation in Browning Drive beside Ebrington Square.
A police spokeswoman said: "Officers came across three men arguing with a fourth. Two of the males began fighting and could not be separated.
"CS spray was used and all those affected received the appropriate treatment. Enquiries are ongoing to establish the cause of the altercation."
Chief Inspector Alan Hutton added: "We worked very closely with the event organisers and other key stakeholders throughout the planning process, particularly around the issue of licensing and public safety
"We did have cause to deal with a number of young people who were intoxicated.
"Some of them were dealt with in a special triage area - however others did require treatment at hospital.
"We did have cause to speak to several people who became disorderly after being removed from the event space. We are following up on a report of an assault on a 15-year-old boy and we arrested a 17-year-old male for disorderly behaviour."
Colin Neill, Chief Executive of Hospitality Ulster, said: "We must ask some serious questions of where those individuals under the age of 18 purchased their alcohol from. We need to ensure that alcohol cannot be purchased by, or for, those who are under-age, and that cheap drink which is sold below cost price is inaccessible to those to whom it is likely to cause most harm."
The event was organised by Legacy Promotions, which has staged MTV concerts in Derry for the past three years.
However, it is understood that the company does not plan to host an under-age event again.
No one from the company was available when contacted by the Belfast Telegraph.