Explosive Ordnance Disposal (EOD) unit rushed to the Criminal Court compound on Ratchadapisek Road after being informed of the explosion sound around 10.25am and found two M-61 hand grenades.
The first grenade misfired outside the court fence. No injuries or damage was reported. The second, falling in the court's car park, failed to detonate.
Bomb squad unit chief Pol Col Kamthorn Auicharoen said the first grenade which went off did not function to its full capacity due to deteriorated gunpowder, while the second did not explode as the assailants did not remove the safety ring.
He said the M-61 grenades were normally used by state agencies and have a 15-29 metre radius of destruction.
Soldiers guarding the court said that they saw two men on a motorcycle wearing helmets and black shirts. They fled the scene after throwing the grenades.
Criminal Court director-general Thaweechai Sanamontree inspected the scene and instructed officials to report the incident to the Supreme Court.
Deputy national police chief Pol Gen Aek Angsananont said the assailants were well-prepared for the attacks as their escape route connected to many other roads. It is believed that they were the same group which earlier created a similar situation.
He said police are now adjusting the security plan with the army, and will increase the number of troops deployed and checkpoints operated, while police will also step up their patrol operations at all court compounds to prevent s recurrence of such an incident.