The extremists came in a white car and hurled the grenade at a busy area of the town just 500 metres from the main police station around 6:30 pm.
Sonitpur district Superintendent of police Sanjukta Parasar, who rushed to the blast site, told PTI that the National Democratic Front of Bodoland-S was behind the attack.
Police, quoting eyewitness, said a white colour car had turned into Itakhola centre towards Seijosa and then the grenade exploded when the business community there was busy celebrating Diwali.
The injured were sent to Dhalaibill Public Health Centre for immediate treatment and businessman Rajesh Gupta, in front of whose shop the blast took place, was sent to Tezpur Kanaklata Civil Hospital.
Also Read
The grenade damaged the assistant manager's vehicle and another car, they added.
The area has been cordoned off and a search operation launched to trace out the assailants.
The NDFB-S struck the lower Assam districts snatching away rifles from forest guards in two camps inside Manas National Park in Baksa on October 22.