Militants hurled the grenade at a road opening party of the army at Sangam Chowk at around 1605 hours, they said.
The grenade missed the intended target and exploded at the roadside wounding four civilians, they said.
Security forces cordoned off the area and launched a search operation to apprehend the attackers, the officials said adding no arrests had been made so far.
No militant outfit had claimed responsibility for the attack, they said.