We have studied the mechanisms of dislocation nucleation from surface defects in silicon submitted to various stresses and temperatures. Molecular dynamics simulations with three classical potentials have shown the existence of two different plastic modes in silicon which can be activated from surfaces. At high temperatures and low stresses dislocations nucleation occurs in the {111} glide set planes, while at low temperatures and large stresses it occurs in the {111} shuffle set planes. The analysis of dislocation cores and kinks shows structures like those well known in bulk silicon. This study supports the idea that plasticity in crystalline Si structures could be governed by dislocation nucleation from surfaces.