Terahertz (THz) pulses with a peak power of $\sim 2\mathrm{kW}$ were generated in a noncollinear phase-matched GaAs crystal at room temperature. Two $200\mathrm{ns}$ pulses from a dual-beam TEA $\mathrm{C}{\mathrm{O}}_2$ laser were used for difference frequency mixing in the crystal. A comb of narrow lines $(\Delta \nu ∕\nu \sim {10}^{-4})$ was obtained in the $0.5–3\mathrm{THz}$ range with a step of $40\mathrm{GHz}$ . By comparing the effective nonlinearity of GaSe with that of GaAs for THz generation, the electro-optic nonlinear coefficient for GaSe was measured to be $d^{\mathrm{eo}}=24.3\pm 10\%\mathrm{pm}∕\mathrm{V}$ . Using simulations we show that a $1\mathrm{kW}\mathrm{THz}$ pulse could be amplified by a factor of $2\times {10}^4$ to a $10\mathrm{MW}$ level in a $2\mathrm{m}$ long single-pass free-electron laser.