High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 millijoule of energy was produced at < 10 terahertz with a laser-to-terahertz conversion efficiency of ~ 0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and plasma wakefields on the time scale of the laser pulse duration. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.