Police said investigators believed the grenades were launched from a pickup truck parked nearby the protest site. The protest, where more than 2,000 people participated, was organised by Kantaphon Sukhumalin, a core member of the anti-government People's Democratic Reform Committee (PDRC).
The first grenade landed in front of the Bang Sai Municipality Office. Glass windows were shattered and two cars were damaged by the explosion.
The second grenade went off next to the Bang Sai Education Development Centre. The office building was partially damaged.