The use of class A drugs by teenagers at the Odyssey was what sparked the Major Incident outside the gig, a volunteer has stated.
Children as young as 13 were so severely ill that they could not speak and their eyes were rolling into the backs of their heads.
The drugs which they admitted taking included heroin, ecstasy and so-called legal highs.
Joe Hyland, chief executive of the SoS Bus, a charity that helps people who get themselves into trouble on a night out, was at the Odyssey on Thursday night as 10,000 people attended a DJ Hardwell concert.
He stressed that there were two different events – the concert inside, which he said was a success, and the chaos outside.
Mr Hyland said what caused the crisis was a combination of a very high concentration of casualties and the severity of the illness.
The charity had two buses and 23 volunteers, but was swamped.
"The event itself truly was successful but the overflow of the young kids that couldn't get in because they were underage or too drunk wasn't properly planned," he said.
He said inexperienced young drinkers led to high numbers of people who fell ill, but also said there was use of class A drugs.
"They have no idea about alcohol and how it affects them; 20% of the alcohol affects them in five to seven minutes and the other 80% goes into your small intestine and can take up to 45 minutes to affect you," he said.
"By the time you have had your fifth/sixth/seventh drink it is only starting to affect you, the rest is still to come.
"When they present themselves to me and they can't walk and can barely speak in the wheelchairs we have, I know that in 50 minutes I am going to be dealing with something much more serious."
At the Calvin Harris concert last December, more than 125 youngsters came to the buses. But on Thursday night there was a higher concentration of people who were more seriously ill in a shorter time period.
"We had in a very short space of time a very, very high number of very seriously ill young people," he said.
"We had 19 young kids on the bus throwing up, crying and you could see them getting worse all the time. But then we saw a pattern and the pattern was nothing to do with alcohol, it was obvious it was drugs.
"The decision was made by the senior doctor to create a Major Incident. We considered this to be a life-threatening situation because if there was a bad batch of drugs those kids lives were threatened – and it did look that way that night."