The suspect Thawat Rassadorn, 29, initially faced three charges: reckless action causing injury to others, having a gun and ammunition without permission and carrying a weapon in public without permission.
Thawat will be taken to Southern Bangkok Criminal Court today to apply for detention pending an investigation.
Vendor Phichit Rungreungpanit, 52, and protester Papassorn Chiangsaen, 46, were injured in the shooting, which occurred near the boxing arena on Rajdamnoen Nok Road.
Thawat reportedly told police he was about to go to sleep in a tent when the gun went off accidentally, leading to the injuries. He said he bought the gun in Prachin Buri and had been at the protest for a week.