Three workers were confirmed killed after a six-storey old building on Sukhumvit road in Phra Khanong collapsed to the ground while it was under demolition.
Three other workers were believed to be trapped underneath the concrete rubble.
Rescue workers are now racing with time to look for those still trapped inside the collapsed building.
Police said they have detained the contractor of the demolition work for questioning.
More heavy equipments and sniffing dogs of the police and the military are now at the scene to look for possible survivors.
Students at an international school near the site were also evacuated for safety reason as rescue workers were searching through the concrete debris for survivors.
Source: Thai Public Broadcasting Service (Thai PBS)