The officers, who are maintaining order around the rally site, arrested Prasert Daosook, 56, who was sleeping in the car when they asked to search the vehicle parked on Liab Khlong Thawi Watthana Road.
They also seized a mobile phone and a briefcase from him. Mr Prasert was later taken to the Crime Suppression Division for questioning, where he denied possesion of the war weapon.
He told police he worked at a garage in the Rangsit area and drove to the United Front for Democracy against Dictatorship (UDD) rally site to collect his wife who was a cook for the protesters. He was sleeping and waiting for her in the car and did not take part in the protest. He claimed he had no idea who owned the impounded AK and ammunition.
Police said they would question him further before pressing charges.