The China-made hand grenade was lobbed at the house of one Khuraijam Loken (48) at Yumnam Huidrom Leikai area last evening, but it did not explode.
Monetary demand was suspected to be the reason for the attack.
Police said all its outposts lying in interior areas had already been put on alert.
Both state and central paramilitary forces have also been asked to step up checking of passers-by and patrolling to intercept the movements of militants, police said.