Police said the drug tested at Weston Park, Staffordshire, contained "really serious chemicals that would have a severe adverse effect".
The chemicals, pentylone and N-ethylpentylone, had been found recently at other music festivals and had made people ill, they said.
Since the event started on Friday, 32 people have been arrested for offences including drug dealing and theft.
A 17-year-old boy was arrested for trying to smuggle 200 tablets in a bag he was carrying into the Staffordshire event.
Supt Martin Brereton, leading the Staffordshire police operation, said "in the first 24 hours [we] have seen very little crime and have proven that our security and policing operation is working well".
He warned festival-goers that taking the powder or crystals would lead to nausea, vomiting, hypothermia and general agitation.
People who had consumed it at other events needed urgent medical attention, police said.
Organisers said the drugs were likely to be sold as MDMA and warned people to be vigilant and contact police if offered the substance.
Staffordshire Police said three men had also been prevented from taking 355 canisters of "laughing gas" into Weston Park.
Jay Z is headlining at the Staffordshire and Essex festivals on consecutive nights.
Emergency crews at Hylands Park, Chelmsford, said most people they had helped were suffering from blisters and dehydration.
A potentially dangerous drug has been found at V Festival.