The People's Democratic Reform Committee (PDRC) leader said evidence found in a deserted building thought to have been the hiding place of the bomb suspect had been handed over to police.
The evidence was retrieved by PDRC guards who refused to let police examine the scene, for fear they could destroy vital forensic information.
Mr Suthep challenged investigators to explain the blast and said he is waiting for answers on the probe from national police chief Pol Gen Adul Saengsingkaew and Pol Lt Gen Kamronwit Thoopkrachang from the Metropolitan Police Bureau.
Evidence handed over to police by the PDRC included three M16 riffles which are not assembled, four walkie-talkies and a cap bearing an "attacking unit" logo, according to Mr Suthep.
Earlier reports said two police emblems, food, food containers and a refrigerator were found in the room where the bulk of the evidence was found.
In his speech to the Pathumwan rally stage, Mr Suthep showed a picture claiming that the cap found at the scene belonged to a police unit in Narathiwat province.
The PDRC leader was unable to hold back tears as called on all demonstrators to pray for a protester who is seriously injured by the blast.
The PDRC will take responsibility for looking after all of those injured in the attack, he added.
Wounded protesters were sent to Chulalongkorn, Huachiew, Ramathibodi and Klang hospitals. Some have already been released.
The bomb is believed to have been an M26 grenade, a grenade cap for which was found at the blast site.