Phetchkasem superintendent Colonel Wutthichai Thaiwat inspected the scene while pawnshop owner Sirichai Asaipanich, 61, who sustained a head injury from being hit by the butt of a firearm, was taken to Kasemrad Hospital in Bang Khae.
The police said Sirichai was opening the pawnshop alone at 8am when three suspects, one wearing a full helmet and the two others wearing face masks, raided the shop.
One suspect carried a gun and another a knife. When Sirichai resisted he was hit in the head and face multiple times until he fell down.
The suspects then opened the glass cabinets and stole 10 items and cash before fleeing on two motorcycles in the Samut Sakhon direction.
Police suspected that the robbers had learned the pawnshop routine to commit the crime when Sirichai was alone. CCTV was being checked and witnesses interviewed, Wutthichai said.