Two men and two women were hit by the gunmen. The attack occurred about 2.30pm near Wat Bua Kwan in Muang district of Nonthaburi province.
The NSPRT supporters were returning to their protest base at Government House after joining the People's Democratic Reform Committee protesters led by Suthep Thaugsuban in a rally at the government office complex on Chaeng Wattana road
It was believed the gunman or gunmen were hiding in a nearby building, PDRC spokesman Akanat Promphan said.
One of the injured victims, a guard who was on a mobile broadcasting truck, was shot in the head and later succumbed to his wounds at the hospital. Bullets also pierced the windows of a bus travelling behind the truck and hit three passengers, Mr Akanat said. Their wounds were not life threatening.