An M79 grenade was fired at the Central Administrative Court late Thursday night, police said.
The attack happened at 10:05 pm and the grenade fell at a tree outside the court on the Chaeng Wattana side. Nobody was injured.
Bomb disposal officers went to check the scene immediately.
The explosion site was near the rally site of the People's Democratic Reform Committee in front of the Government Complex.
Luangpu Buddha Isara, the monk who led the rally at the Government Complex, and his guards also went to check the explosion scene.