Removing bodies from the wreckage took hours, the Times of Zambia reported. Both the truck and bus were crushed in the collision.
The truck driver, his assistant and the bus driver were all killed.
Andrew Mbewe, Central Province deputy commissioner of police, and Chibombo District Commissioner Felix Mangwato said the truck, believed to be from the Democratic Republic of Congo, struck a Toyota Hilux and then hit the bus.
The driver of the Toyota, also from outside Zambia, was detained, although some witnesses said the truck driver was to blame for the crash.
Alex Mayela, the postal service operations manager, said 65 passengers boarded the bus in Ndola. It was bound for Lusaka.
He said the bus had an authorized capacity of 70 passengers.