Fourteen young concertgoers have been admitted to hospital and many more were treated at the scene during an incident in Belfast, health authorities said.
Doctors and ambulances were called to help up to 50 rave party goers outside the Odyssey Arena. But a spokeswoman for Belfast Health and Social Care Trust, which represents the city's hospitals, said no major incident had yet been declared.
Odyssey general manager Adrian Doyle said they arrived at the venue already drunk.
"It is our understanding that a number of patrons arriving at the Odyssey Arena to attend a concert by Dutch DJ act Hardwell have required medical attention, having already been intoxicated on their arrival at the Odyssey Complex," he said.
"These patrons were not granted entry to the Odyssey Arena, and were treated by the Ambulance Service outside."
He said the concert continued as normal inside the arena.
"As with any concert, the safety of our patrons is our absolute priority at all times and the event control team is confident that we have sufficient staff on site to maintain a safe and enjoyable concert," he added.
The Northern Ireland Ambulance Service (NIAS) has declared a major incident after up to 50 young people attending the event at the entertainment centre on the edge of the city centre became ill. Some may have consumed drugs.
A spokesman for the ambulance service said: "We believe the majority arrived suffering from too much alcohol.
"We declared a major incident because of the number of resources we had to deploy to the scene."
Hardwell was named the world's number one DJ in the 2013 DJ Mag Top 100 DJs Poll.
His hits include Spaceman, Three Triangles (Losing My Religion) and Apollo.
Last month dozens of fans camped overnight or queued for hours on a Belfast street in pouring rain for a ticket to his show.
A Belfast health trust spokeswoman said local hospitals had been put on standby.
"There has been no major incident set up," she said.
Colin Neill, chief executive of Pubs of Ulster which represents local bar owners, said he was concerned.
"Pubs of Ulster would reiterate that alcohol should be consumed responsibly and safely. Pre-loading at home before going out is dangerous and can lead to unintended consequences with people putting themselves at great risk," he said.
"Parents must stop under-18s getting access to drink and licensees should check ID and exercise extreme caution at all times.
"We urge pubs in Belfast to be extra vigilant this evening - be on the lookout for persons underage and do not admit young people possibly coming from the Odyssey who have had too much to drink."
Judith Cochrane, Alliance Party MLA at the Northern Ireland Assembly, said: "I am deeply concerned for the wellbeing of these young people. My thoughts are with them at this time. I hope they will all be able to make full recoveries.
"This incident is believed to have been caused as a result of alcohol and drugs. It should act as a warning to other young people of the dangers posed by drugs and alcohol.
"I would like to pay tribute to the work of the ambulance and hospital staff who have responded to this incident."
A hospital spokesman said the patients were in a stable condition in the Royal Victoria and Mater hospitals and would be kept in overnight for monitoring.
A Police Service of Northern Ireland (PSNI) spokesman confirmed that the young people that attended the Odyssey were believed to be intoxicated. They were initially treated by volunteers from the SOS Bus NI charity.
A helpline for concerned parents has been established at 02890638131.