The two assailants went to the post office around 6.30 PM when its Post Master Rabindra Kumar Biswas was counting the day's collection of small savings before depositing the amount in the nearby sadar police station, they said.
Stating that the gunmen came in the pretext of depositing money, the police said when the Post Master refused to accept their money as the post office had closed for the day at 4 PM, the assailants opened fire seriously injuring the PO's guard.
The guard identified as Bengaram Rai, 55, and was admitted to Dhubri Civil Hospital.
District Additional Superintendent of Police Indranil Barua rushed to the spot and a search operation was launched to nab the culprits.