Thalang police had set up a checkpoint at the Khaw Lan-Baan Bangjo junction in Srisoonthon to check vehicles, and at 1:30am a Hona Jazz was stopped and the three people inside were asked to step outside while the vehicle was searched.
During the search officers discovered a live M67 hand grenade hidden on the back seat under a pile of magazines.
The driver of the Honda Jazz, Songchai Nienhom, 38, from Krabi and his two passengers, Nampon Hohhow, 34, from Maha Sarakham and Ms Wijitra Chaweein, 34, from Nakorn Sawan were taken to Thalang Police Station for questioning.
Police Lt Col Prasan Toworn said that the trio told officers that they were heading back home after they had been to discuss a financial situation with the J Karaoke Bar in Srisoonthon and they had taken the grenade with them for protection in case the situation got out-of-hand.
All three have been charged with illegal possession of an explosive war device.