The Explosive Ordnance Disposal (EOD) unit, police from Forensic Science Office and Thung Song Hong subdistrict have inspected the scene which was attacked at around 11pm last night.
Investigators said the grenade ripped through the tent and exploded inside as anti-government protesters stayed during the night.
Fragments were found scattered around the site and police believed it was an M-79 grenade.
The victim was identified as Yuthanan Songthong, 25, a PDRC guard. He was wounded on his torso and leg and was sent to Mongkut Wattana General Hospital.
The witnesses said they heard the explosion at around 11pm and believed that the M-79 was launched from the outbound side of Chaeng Wattana Road, near the CAT Telecom office.