Police said a group of rally guards of the People's Democratic Reform Committee (PDRC) detained Pol Sgt Pat Phetarwut and took him to Samran Rat police station, together with one gramme of crystal meth and one 9mm pistol.
Pol Sgt Pat, a Public Relations Division officer, told investigators that his job was to oversee the Police Museum on nearby Ratchadamnoen Nok Road.
He had parked his motorcycle at the rally site and it was later hauled away by one of the PDRC guards.
"When I asked the guards to return the vehicle and told them that I'm a police officer, the guards searched and assaulted me before bringing me to the police station," Pol Sgt Pat said.
He said he carried the pistol with him because he did not want to leave it at home and that he got the crystal meth from a friend.
Pongpan Sema, a PDRC guard, told police that Pol Sgt Pat threatened to draw his gun on some protesters and chased them. The guards then decided to detain and search him.
Pol Sgt Pat was given a urine test for drugs and the result was positive, police said.
He was charged with possession of drugs and drug abuse.
Previously, police arrested three PDRC guards on drug charges. The anti-government group then accused police of trying to link them with drugs.
Police then released photos of the drug suspects who allegedly admitted that they abused "4x100", an illegal mixture made by mixing kratom leaves and cough syrup, before joining the protest in Ratchadamnoen. The mixture is popular in the South.