A critical incident investigation has been launched following the death of a teenager at Knockout Circuz music festival in Sydney’s inner west overnight.

Police have been told that shortly after 10.30pm yesterday, an 18-year-old man was running on Riverina Avenue, Sydney Olympic Park, near the entrance to Hall 2.

The man appears to have tripped over the feet of several people who were sitting in the area, and fell to the ground.

After suffering head injuries in the fall, the man was assisted by security guards.

Police were called to the incident after the man, who was drifting in and out of consciousness, became violent with security personnel.

The man was restrained and treated before being taken to Westmead Hospital.

Overnight the man’s condition deteriorated and he died about 1am.

A critical incident team is now investigating all circumstances surrounding the incident.

That investigation will be subject to independent review and all information will be provided to the Coroner.

Police are urging anyone with information about the incident to come forward and contact Crime Stoppers on 1800 333 000.

A teenager has died after falling over at Knockout Circuz music festival. (9NEWS)

The incident follows five arrests of people taking illicit drugs at the festival as part of Operation Clinker.

Approximately 16,900 people attended the event. During the operation, police searched 298 people and charged 55 for possessing a prohibited drug, issued six supplying cannabis cautions, charged two people for trespassing and one for failing to quit a licensed premises.

Of those arrested, five people have been charged with possessing MDMA.

A 17-year-old girl was charged after police found 379 MDMA capsules on her, a 22-year-old man was charged after police found 188 MDMA capsules, a 19-year-old woman was charged after police found 90 MDMA capsules, a 19-year-old woman was charged after police found 40 MDMA capsules, and another 17-year-old girl was charged after police found 41 MDMA capsules.