Phahon Yothin police and an explosive ordnance disposal (EOD) team were sent to the scene to examine the explosive.
The police used a car tyre to cover the unexploded grenade and cordoned off the area to keep people, many of whom were panicking, away.
A large number of court officials had showed up at their offices Sunday when a new cooperatives committee was to be elected.
It was initially believed one of the two courts was the target for an attack.
Police were examining the footage of surveillance cameras in the area to find clues to the culprits.
Video by Pattarapong Chatpattarasill.