Dyfed-Powys Police and military personnel have been called to the event near the Drovers Arms which is taking place on Ministry of Defence land.
Police said they were working with military and local authorities to minimise disruption and inconvenience to the community.
They have blocked roads in the area surrounding the rave.
A spokesman said the B4519 at the Upper Chapel end and the Garth end was blocked and others roadblocks were in place in the area.
Police have not said how long the event has been going on for…
