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A fast algorithm for accelerating the time-marching solution of time-domain integral equations pertinent to the analysis of free-space electromagnetic scattering from perfectly conducting, platelike and uniformly meshed structures is presented. The matrix-vector multiplications required by the time-marching scheme are accelerated by use of the fast Fourier transform (FFT). This acceleration is achieved in a multilevel fashion by hierarchically grouping sparse interactions to extract denser pieces that are efficiently evaluated by the FFT. The total computational cost and storage requirements of this algorithm scale as O(NtNslog2 Ns) and O(N1.5), respectively, as opposed to O(NtNs2) and O(Ns2) for classical time-marching methods (Ns and Nt denote the total number of spatial unknowns and time steps, respectively). Simulation results demonstrate the accuracy and efficiency of the algorithm.