One of two small pipe bombs exploded on the pavement near the Democracy Monument Wednesday evening, the day before the promulgation of the new constitution.
Police said the device had exploded inside a dustbin on Ratchadamnoen Avenue in front of the old Government Lottery Office.
Two street cleaners were taken to hospital with ringing in their ears, but there were no other injuries, police said.
"It's not an act of terrorism or an act to stir unrest," national police chief Pol Gen Chaktip Chaijinda said.
Earlier report
Two persons were slightly injured when an explosive device went off in front of the former headquarters of the Government Lottery Office on Wednesday night.
Chana Songkram police chief Pol Col Pitak Sutthikul reported the incident at 8pm.
Two workers of the Bangkok Metropolitan Administration were slightly injured and sent to Vajira Hospital.
Explosive ordnance disposal officials and police were at the scene and reportedly found another homemade bomb in a trash bin in the area.