Energetic ions are emitted from a metal (copper) surface irradiated with femtosecond laser pulses even at extremely low laser fluences. The nonthermal interaction of the pulses with the metal surface is investigated by energy spectroscopy of ions emitted from the surface. Singly charged ions with energies of 180 eV are produced at low fluences of 80 mJ/cm(2), and the ion energy spectrum does not follow a shifted Maxwell-Boltzmann distribution. As the mechanism of ion acceleration, Coulomb explosion of nanoparticles on the target surface is proposed. This mechanism is supported by the fairly good agreement between calculated and experimental ion energy spectra and by the relationships of ion energy and ion emission amount with self-organized nanostructures formed on the irradiated surface.