No one was injured in the blast, they said.
Two motorcycle-borne militants of the National Democratic Front of Boroland (Songbijit) hurled a grenade in Chapaguri Chowk around 8.40 AM.
As both police and public chased the ultras, they fled leaving their bike.
In the second incident, the insurgents threw a grenade at a road at Dumgaon under Runikhata police station around 12 noon.
The blast created a crater on the road, police said.
Investigations are on and security measures stepped up in view of the NDFB(S) along with ULFA (Independent) and four other North East underground outfits calling a boycott of the Independence Day and a general strike tomorrow in the entire region from 1:30 AM to 5:30 PM.