The Northern Ireland Ambulance Service has declared a major incident after up to 60 young people attending a concert in Belfast became ill.
A doctor and several ambulances are at the scene of the Odyssey Arena.
A number of people have been taken to hospitals across Belfast to be treated for the effects of drugs and alcohol.
They had been attending a dance event headlined by Dutch DJ Hardwell.
Last month dozens of fans camped overnight or queued for hours on a Belfast street in pouring rain for a ticket to his show at the Odyssey Arena.
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.
Michael Copeland, an Ulster Unionist member of the Northern Ireland Assembly, said an eyewitness described the scene as "horrendous".
"It was an event for 16-18-year-olds and around 60 young people have gotten themselves into difficulties with alcohol.
"A number have been taken to hospital."
A spokeswoman for Belfast Health and Social Care Trust, which represents the city's hospitals, said no major incident had been declared.
"The Royal, the Ulster and the Mater have been on standby about the issue at the Odyssey," she said.
She is awaiting an update from people "on the ground".
She added it had not transpired into a major incident so far.
"There has been no major incident set up."
Odyssey Arena general manager Adrian Doyle said he was aware that a major incident has been declared by the Northern Ireland Ambulance Service outside the Odyssey Complex.
"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. Seven patients have now been transferred to hospital by ambulance."
He said the concert was continuing 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.
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, 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."
Northern Ireland Ambulance Service spokesman John McPoland 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."
DUP MLA Robin Newton said: "With up to 10,000 youngsters arriving at this event and with many arriving in bus loads from across Northern Ireland there is a need for effective communication on the wellbeing of our young people.
"I am grateful that the emergency services have put their plans into action and that young people are being treated at the scene or in local hospitals."