BlueSky television showed pictures of loaded guns, food, styrofoam food containers with delivery names written on them, towels and other evidence of a long stay, including a working refrigerator, in a room inside the unoccupied building after a search by security guards of the People's Democratic Reform Committee (PDRC) and authorities.
A red cap with the inscription "Attacking unit'' was also found in the room, the report on BlueSky showed.
Although the building, near Supachalasai stadium in Pathumwan, appeared unoccupied, electricity had not been cut off and the refrigerator was still working.
"This is scary,'' former Democrat MP for Rayong Sathit Pitutecha said at the scene.
The hunt for a bomber was continuing.
Protesters saw a man in the building about 1pm, around the time the attack occurred. The Erawan Emergency Centre's latest report said 31 people were injuried.
Only selected reporters were allowed inside the building by PDRC guards.