Fourteen seriously wounded people were taken to a hospital in the nearby town of Dibrugarh, Mishra said.
Police blamed the blast on the United Liberation Front of Asom, a separatist group that has been fighting for an independent homeland in the area for more than two decades.
Mishra said the attack came as the rebel group observed its "Army Day," commemorating the date it set up its armed wing in 1981.
Several rebel groups are fighting for autonomy or independence in India's northeast. The militants say India's national government exploits the region's rich natural resources while doing little for the area's indigenous people, most of whom are ethnically closer people in nearby Myanmar and China than to the rest of India.