The incident occurred when the wedding party of the groom's side was returning to Guwahati after completion of marriage in Jamlai, a police official said.
Stating that the groom and the bride were safe as they were travelling in another vehicle, he said, the five, including the child, were killed on the spot.
Of the 20 injured, 15 have been shifted to the Gauhati Medical College Hospital and the rest were admitted to the Chaygaon Public Health Centre.
According to the police, they had seized the truck but the driver is absconding.
The bus was badly mangled under the impact of the collision, police added.