More than 100 young people were treated by paramedics for the effects of alcohol and, in some cases drugs, at a concert in Belfast.
They were attending a DJ Hardwell gig at the Odyssey Arena, on Thursday.
A charity worker who helped to treat people at the scene told the BBC it was "like a disaster zone".
On Friday, a DJ Hardwell gig in Edinburgh was cancelled when its licence was withdrawn in the wake of the Belfast incident.
Three of the 17 people who were hospitalised came from inside the Odyssey Arena
Police are investigating and are to meet Odyssey management. They arrested three people and seized a small quantity of drugs outside the venue.
'Buckets of vomit'
Sources said police will also investigate claims that many underage teenagers were able to buy alcohol as well as being allowed in while clearly drunk.
Ten thousand tickets were sold for the event, and police said about 300 young people were refused entry to the venue because they were either underage or intoxicated.
A total of 108 people, including some very young teenagers, needed medical help both inside and outside the venue.
The Northern Ireland Ambulance Service (NIAS) declared the scene outside the concert as a "major incident" at about 20:00 GMT on Thursday.
They said this was due to the number of resources they had to deploy in order to treat the patients.
A Police Service of Northern Ireland representative said they would review the incident with management at the Odyssey and that CCTV footage would form part of their investigation.
A statement from the Odyssey Arena general manager on Thursday night said those who were intoxicated were not allowed into the arena and were treated by paramedics outside the venue.
However, on Friday it was confirmed that more patients were treated inside the venue than outside its doors.
Sixty-eight people were treated inside the premises by medical staff from Proparamedic, a private firm hired by the Odyssey.
A further 40 people were treated outside the building by paramedics from the Northern Ireland Ambulance Service.
Seventeen people were taken to hospital. Three of them have since been discharged and the remaining 14 are in a stable condition.
Charity worker Joe Hyland, who runs the SOS Bus NI voluntary service, said: "I was dealing with kids with buckets of vomit and they were crying - it was a mess."
Five ambulances and a number of rapid response NIAS vehicles were called to the venue.
The 17 young people who were very ill were treated in accident and emergency departments at two hospitals in the city, the Royal Victoria and the Mater.
Extra staff were called into the hospitals to cope with the situation.
Mr Hyland, chief executive of the SOS Bus NI charity, told BBC Radio Ulster: "It was like a disaster zone, but that was primarily because of one factor - it was a compression.
"There was a period of about 45 minutes where we had something like 19 young people - 14, 15 years old, who were extremely ill.
"Some of them were very, very ill - life-threateningly ill, where they had over-indulged.
"We recognised an increasing threat to their life. We started to bring in paramedics, the ambulance service."
Mr Hyland said that when paramedics arrived they "recognised the numbers were too big" and they had to call a major incident.
"Their resources were limited, they had something like seven ambulances tasked for the evening and when they realised that they were in a danger of not being able to cope they brought in their senior people, who I think made the very best decision," Mr Hyland said.
The charity worker said calling a major incident had allowed paramedics to get the resources they needed "to protect life".
However, he also said that the staff at the scene "were really worried, was there perhaps a bad drug out there?".
Northern Ireland Ambulance Service spokesman, John McPoland, said many of those treated by paramedics were already intoxicated when they arrived at the venue.
He said paramedics treated two people who were "in an unconscious state".
"We believe the majority arrived suffering from too much alcohol," Mr McPoland added.
The PSNI representative said: "Over the course of the evening, approximately 300 young people, some of whom had travelled to the venue by coach or other group transport, were showing signs of intoxication or were not the correct age and were not permitted entry to the venue".
At the scene, officers arrested a 16-year-old girl, an 18-year-old man and another man aged 21.
The teenagers were both detained on suspicion of assault and resisting police. The 21-year-old was arrested disorderly behaviour.
A spokesman for the Odyssey Arena, Adrian Doyle, said on Thursday: "It is our understanding that a number of patrons arriving to attend a concert by Dutch DJ act Hardwell have required medical attention, having already been intoxicated on their arrival at the complex.
"These patrons were not granted entry to the Odyssey Arena, and were treated by the ambulance service outside."
Among those attending the concert was Scott McBride, who said a number of people were being sick as he arrived at the venue.
"Everywhere you looked, you were guaranteed to see someone who was ill," he said.
"One of the guys I saw was sitting in his own vomit against a wall and struggling to keep his eyes straight."
Another concert-goer, 16-year-old Reece Dempster, told BBC Radio Ulster: "When I left, there were fights and sirens everywhere, ambulances left, right and centre and police.
"There were people throwing up beside the Odyssey and around the corner, just throwing up everywhere.
"There were a few fights inside as well."
Another concert-goer said he left the venue after witnessing what he described as a "brawl".
"When the fight broke out everyone moved back and people were getting pushed everywhere. It was dangerous - but security were there within 20 seconds.
"Outside, it was crazy. There were parents waiting, people off their heads not knowing where they were going - basically bedlam outside," the young man said.
Many parents of the young people who were attending the over-16s concert could not get in touch with their children while the event was taking place.
As the concert was coming to an end at about 23:00 GMT, large numbers of concerned parents arrived at the Odyssey and waited outside.
On Friday, the Odyssey Arena general manager Doyle said thorough pre-planning had taken place with the PSNI and Belfast City Council.
"Both the PSNI and Community Enforcement Teams from Belfast City Council were on the ground at the event confiscating alcohol that was in the possession of patrons as they arrived at the Odyssey," he added.
"In anticipation of any patrons being turned away at the door for any reason, the support services of the SOS Bus were also deployed.
"High levels of staff were employed to manage queues at the door. Searches were carried out on the door by a team of Security Industry Authority (SIA) accredited personnel, with two PSNI sniffer dog teams also deployed.
"Large numbers of patrol and response staff were on the concourses and in the main hall throughout the evening and supervisors checked toilet facilities on an ongoing basis.
"Stringent ID checks were in place. All alcohol sales were stopped at 9pm."