A small device was thrown into a canal and exploded near a busy train station in central Bangkok on Tuesday afternoon, police said, a day after a bomb killed 20 at a religious shrine.
"No one was killed or injured. Police are at the scene to investigate what kind of device it was," an officer at Yanawa police station, who asked not to be named, told AFP.
He said the incident took place shortly after 1pm (0600 GMT) near the Saphan Taksin BTS skytrain station.
CCTV footage of the incident posted by Thairath TV showed commuters scurrying for cover over a footbridge after the device exploded in the canal, sending a large plume of water into the air.
The Yanawa police official said officers believed the device was thrown from a road and rail bridge which spans Bangkok's Chao Phraya River and overlooks the canal below.
The luxury Shangri-La hotel, the Mandarin Oriental and the French embassy are close to Saphan Taksin station.
No one claimed responsibility for the incident.
Bangkok is a city on edge after Monday night's bomb attack, described by junta chief Prayuth Chan-ocha as the "worst ever attack" on Thailand.
At least eight foreigners are known to have died in the attack alongside five confirmed Thai deaths. The rest have yet to be identified.
More than 100 other people were injured as the blast left body parts strewn across the pavement, alongside shattered windows and mangled motorcycles.