Dyfed-Powys Police were notified of an illegal rave taking place at the Usk Reservoir area at around 9.30pm last night.
Officers attended to find approximately 200 people in attendance, with 70 or 80 cars on the site.
They exercised the powers to prevent the situation escalating, including closing roads leading to the site with the support of local authority highways.
The illegal gathering began winding down at around 6am.
One person was taken to hospital by ambulance.
No arrests have been made, however, enquiries are ongoing.
This morning a multi-agency meeting was held with key partners affected by the illegal gathering.
We would urge people living in rural communities, including farmers and landowners, to report anything suspicious to police.
The information would help police respond swiftly as illegal raves arise and hopefully prevent them from happening or at the very least allow police to respond before they become established.
If you suspect an illegal rave is being planned in your area, you can report it to us online: https://bit.ly/DPPContactOnline, by emailing firstname.lastname@example.org or by calling 101.