A power assist module includes a battery powered motor mounted on a manually operated vehicle, such as a bicycle. The power assist module is fixedly mounted on the bicycle and can be easily removed without the need for tools, so that the bicycle can be manually operated in a conventional manner when so desired without carrying the extra weight of the power assist module. The power assist module includes a motor assembly which is operated through a gear reduction assembly to drive a crank drive gear mounted in conjunction with a crank arm of the bicycle.