A loud bang believed to be from an explosive device was heard at the Criminal Court on Ratchadaphisek Road on Saturday night.
The incident at 8.20pm resulted in slight damage at a parking lot inside the court compound, police said. No injuries were reported.
Maj Gen Apirat Kongsompong, deputy commander of the First Army Region, who is duty bound to oversee security in Bangkok, joined police from the Explosive Ordnance Disposal (EOD) unit and Phahon Yothin station in an investigation at the scene.
Pol Col Kamthorn Ouicharoen, superintendent at the EOD Unit of the Metropolitan Police Bureau, said police had not concluded what kind of explosive device was involved. The area was sealed off for officers to collect evidence.
An eyewitness said she saw a black pickup truck heading for Ratchayothin intersection after hearing the noise.
No injuries and casualties were reported from the scene.