Sgt Jakkapong Sriprom, 27, of Directorate of Joint Communications, and Sattawat Chaiyaporn, 24, were travelling in a pickup truck when they were stopped for a search at a police checkpoint on the MCOT intersection on Rama 9 Road.
A search of the vehicle revealed an MP5 submachine gun, a 9mm Glock handgun, more than 70 9mm and .223 calibre bullets and a giant firecracker. They were found in a shoulder bag placed on the back seat.
Other items seized from the pair included two balaclavas, a walkie-talkie, a torch and a Lumpini Park map.
Sgt Jakkapong, the pickup driver, and Mr Satawat, denied they were PDRC security guards or involved with any political protest activities.
The men were taken to Makkasan police station for questioning. Sgt Jakkapong told police he and his friend Mr Sattawat left Chachoengsao province to return home in Pracha Songkhro in the city's Din Daeng area and the impounded weapons belonged to his elder brother.
He had the Lumpini park map because he was assigned by the Internal Security Operations Command (Isoc) to “gather intelligence” at the PDRC main rally site.
Police said the men would initially be charged with carrying guns, ammunition and an explosive item without permission.