  A novel scheme for generating powerful terahertz (THz) radiation based on
laser-solid interactions is proposed. When a $p$-polarized femtosecond laser
impinges obliquely on a plane solid target and the target partially blocks the
laser energy, surface electrons are extracted out and accelerated by the laser
fields, forming a low-divergence electron beam. A half-cycle THz radiation
pulse is emitted simultaneously as the beam passes by the edge of the target,
due to coherent diffraction radiation. Our particle-in-cell simulations show
that the relativistic THz pulse can have an energy of a few tens of millijoule
and the conversion efficiency can be over 1$\%$ with existing $\sim$J level
femtosecond laser sources.
