The shooting occurred early Saturday morning when an unidentified group of gunmen fired M16 rifles at Chamai Maruchet Bridge near the rally site of the Network of Students and People for Thailand Reform (NSPTR).
Pol Col Napon Kladkhemphet of Nang Lerng police station said that the attack kills one NSPTR security guard, identified as Jamriang Jitwat, and wounded three others. He added that all the injured victims have been hospitalised.
A probe is underway and police officers are coordinating with NSPTR leaders to inspect the scene.
An initial investigation found that the group of assailants parked a sedan near the rally site and sprayed bullets at guards and protesters before speeding away from the scene.