The "dummy cameras" were the result of a budget shortfall in implementing the policy of then governor Apirak Kosayodhin to install an initial batch of 10,000 surveillance/traffic cameras, deputy Bangkok governor Theerachon Manomaiphibool said.
A posting on the online community at www.Pantip.com had uncovered the discrepancy.
He did not say whose idea it was to install the dummy ones, or how many units were short of the total of 10,000.
The measure was designed to discourage rioters and protesters from committing crimes during political rallies in the past few years, he said.
Suthon Anakul, a senior Bangkok Metropolitan Administration official, said the BMA had lied to the public when it said the footage requested by police or those in need was not available because certain cameras were broken or out of use.
"But now that the public is aware of the dummy cameras' existence, I would like to apologise for this," he said.
"Installation of the remaining units in the first 10,000 is underway, and there will be 20,000 cameras installed by next year," he added.