Thai police on Thursday said they had questioned and freed one man who handed himself in after being seen on CCTV at the Bangkok shrine moments before a deadly bomb blast, but the prime suspect remains at large.

A Thai man wanted for questioning after he was seen on security camera footage "met police and was released", national police spokesman Prawut Thavornsiri told reporters, adding a second man from China in the same shot had already left the country -- but neither were "likely involved".

The main suspect, believed to be a foreigner in a yellow shirt seen depositing a backpack at the scene was still at large, he added.