In Assam, 12 persons including three police constables were injured in a grenade blast at Doomdooma in Tinsukia district this evening. IGP L R Bishnoi said that the two, motor cycle -riding men hurled the grenade at Gandhi Murti area of Doomdoma.Three injured were rushed to the Tinsukia civil hospital. Senior officials arrived at the spot. Security forces have intensified search operation to nab the culprits.