The incident occurred when the bus hit the railing of a small bridge before rolling down into a roadside ditch at around 1.30 AM, Superintendent of Police Arabinda Kalita said.
Eight persons, including the bus driver, died on the spot, while a woman succumbed to her wounds in hospital, he said.
Of the 28 injured, five critically wounded persons were referred to the Gauhati Medical College Hospital, Kalita said.
Four others were allowed to leave the government-run civic hospital at Nagaon after first aid as their injury was not serious, the SP said.
The surviving passengers said the chartered bus was carrying 35 people from Lakhimpur district to Guwahati so that they could board a train this morning for their onward journey to Delhi.