[en] The critical temperature and the nature of the QCD finite temperature phase transition are determined for N_f=2 dynamical flavors of nonperturbatively improved Wilson fermions. The calculations are performed on large lattices with temporal extents N_t=12, 10 and 8, and lattice spacings down to a=0.075 fm. We find the deconfinement and chiral phase transitions to take place at the same temperature. Our results are in broad agreement with a second order phase transition in the chiral limit. The critical temperature at the physical quark mass is found to be T_c=174(3)(6) MeV. (orig.)