Fans were left upset after fights broke out among some in the crowd at Newmarket, despite the event being billed as a 'family event'.
Cambridgeshire police say they are investigating reports of common assault at the gig.
A third teenager has been arrested in connection with an attack on a man in his 40s in Harlow.
A 15 year-old boy is in custody after being arrested on suspicion of attempted murder.
The attack took place in Southern Way at around 9.30pm on Friday, June 23.
The victim had been walking towards Staple Tye when he got into an argument with a group of people which was described to police as being mainly boys.
He suffered a stab wound to his chest. He has been taken to hospital and his injuries are not believed to be life-threatening.
Officers arrested two boys, aged 13 and 15, on suspicion of assault. They are currently in custody.
