Pol Col Kachornpong Jitphakphum, commander of the Thonglor police, said the female worker with Bangkok Metropolitan Administration injured her right thigh above the knee and was rushed to Chulalongkorn Hospital.
The explosion occurred on Sukhumvit Soi 40.
Kachornpong said the worker was sweeping the road in front of Pathum Khongkha School and found a cloth bag hidden in small trees on the roadside.
The worker carried the bag with her but dropped it three times, so she decided to open the object – wrapped in black adhesive tape – and it exploded.
Kachornpong said it might have been a ping-pong bomb put there by students and may not be a device to cause public alarm.
Police probe also found several knives, which were suspected to belong to students who would carry them for self-defense or brawls with rival youths,.