Relativistically colliding plasma is modeled by particle-in-cell simulations in one and two spatial dimensions, with an ion-to-electron mass ratio of 400 and a temperature of 100 keV. The energy of an initial quasi-parallel magnetic field is 1% of the plasma kinetic energy. Energy dissipation by a growing wave pulse of mixed polarity, probably an oblique whistler wave, and different densities of the colliding plasma slabs result in the formation of an energetic electromagnetic structure within milliseconds. The structure, which develops for an initial collision speed of 0.9c, accelerates electrons to Lorentz factors of several hundred. A downstream region forms, separating the forward and reverse shocks. In this region, the plasma approaches an energy equipartition between electrons, ions, and the magnetic field. The electron energy spectrum resembles a power law at high energies, with an exponent close to −2.7, or . The magnetic field reflects upstream ions, which form a beam and drag the electrons along to preserve the plasma quasineutrality. The forward and reverse shocks are asymmetric due to the unequal slab densities. The forward shock may be representative for the internal shocks of gamma-ray bursts.