The Bangkok Metropolitan Administration (BMA) Erawan Medical Centre did not give the name of the victim but said that about 5.20am a 34-year-old man was wounded in an explosion at Lumpini Park, the People's Democratic Reform Committee (PDRC) rally site.
The victim was sent to Chulalongkorn hospital for treatment
Earlier, an M79 grenade was shot into a security check point of the anti-government rally site at Chaeng Wattana but caused no injuries.
The police bomb disposal unit and army bomb experts jointly inspected the scene, in front of a TescoLotus department store, after an explosion at about 11.30pm.
Police said that the initial investigation found M-79 grenade fragment and that the projectile was launched from the road along Klong Prapa canal.
The target of the bomb was thought to be a police-military security checkpoint 50 metres from the blast, about one kilometre away from the rally site of the PDRC.