Dyfed-Powys Police carried out search warrants at the Leighton Arches caravan park in Welshpool on Monday, where three people were arrested.
Up to 80 officers were involved in the operation, during which what are believed to be class A and B drugs were seized.
A further man has been arrested on suspicion of conspiracy to supply cocaine in connection with the investigation. The 24-year-old remains in police custody.
Police said Michael Power, 19, and John Paul Power, 25, have been charged with conspiracy to supply cocaine and cannabis.
Patrick David Stokes, 32, has been charged with conspiracy to supply cocaine.
The men have been remanded in police custody.
Detective Inspector Richard Lewis said: “This was a significant operation as part of our commitment to eliminate the risk caused by illegal substances in our communities.
“Our investigations are ongoing, and we will provide updates as and when appropriate.”