The victims were returning from Sidhauli after taking part in a Tilak ceremony on the tractor-trolley when the bus belonging to Sahara group hit it from behind in Ataria police station area, they said.
The deceased were identified as Bharat (55), Vaid (40), Guddu (40), Chaitu (55), Suneel (25) and Manohar (35), all residents of Ataria.
Over 20 people were injured out of which 6 were in a serious condition. They were rushed to trauma centre Lucknow, police said.
The bodies have been sent for post-mortem examination.