Police say they have made 11 arrests at an illegal rave which is continuing in the Epynt mountains, near Brecon.
Hundreds of people have gathered near Drovers Arms
Dyfed-Powys Police and military personnel were called on Sunday to the event, near Drovers Arms, which is being held on Ministry of Defence land.
Police said they worked with military and local authorities to minimise disruption to the local community.
Officers said on Monday that 11 arrests had been made on suspicion of drugs offences.
On Sunday, Dyfed-Powys Police said between 300 and 500 people were at the event.
The force confirmed on Monday that the rave was continuing and that 11 arrests had been made.
Those arrested are being held at Brecon Police Station.
The area is used by the Ministry of Defence
The MoD owns 30,000 acres of land at Epynt which it uses for training.
Meanwhile, farmers in Pembrokeshire were warned to be on lookout over the bank holiday weekend for early warning signs of possible illegal raves.
Authorities in the county had admitted they were not properly prepared when more than 500 people overran farm land at Merles during a weekend in 2003.