Flight from Tomorrow by H. Beam Piper

It was broad daylight when he woke, and there was a strange throbbing sound; Hradzka lay motionless under the brush where he had slept, his blaster ready. In a few minutes, a vehicle came into sight, following the road down the mountainside.

It was a large thing, four-wheeled, with a projection in front which probably housed the engine and a cab for the operator. The body of the vehicle was simply an open rectangular box. There were two men in the cab, and about twenty or thirty more crowded into the box body. These were dressed in faded and nondescript garments of blue and gray and brown; all were armed with crude weapons -- axes, bill-hooks, long-handled instruments with serrated edges, and what looked like broad-bladed spears. The vehicle itself, which seemed to be propelled by some sort of chemical-explosion engine, was dingy and mud-splattered; the men in it were ragged and unshaven. Hradzka snorted in contempt; they were probably warriors of the local tribe, going to the fire in the belief that it had been started by raiding enemies. When they found the wreckage of the "time-machine", they would no doubt believe that it was the chariot of some god and drag it home to be venerated.