LONDON (AFP) - The Northern Ireland Ambulance Service on Thursday declared a major incident after 60 youths required treatment for the suspected effects of drugs and alcohol at a concert in Belfast.

An ambulance worker said two of the patients were found unconscious and five required hospital treatment after becoming ill at the performance by Dutch dance artist DJ Hardwell in the city's Odyssey Arena.

He told Sky News that many of youths were showing general signs of feeling unwell and were vomiting frequently.

A spokesman for the Odyssey Arena, Adrian Doyle, said: "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."

Local politician Michael Copeland said an eyewitness described the scene as "horrendous".

"It was an event for 16-to-18-year-olds and around 60 young people have gotten themselves into difficulties with alcohol," he explained.

Hardwell was recently named the world's number one DJ in the 2013 by DJ Mag.