  The cross section of top pair production in hadronic collisions to O(alpha
\alpha_s^2) is calculated within the General 2-Higgs-Doublet Model and the
Minimal Supersymmetric Standard Model. At the parton level the O(alpha)
one-loop corrections to the main production mechanisms, quark-antiquark
annihilation and gluon fusion, significantly modify the Born-cross sections: in
the threshold region \sqrt{\hat s}>~2 m_t they are enhanced up to 50 % and with
increasing cm energy \sqrt{\hat s}, they can be reduced by up to the same order
of magnitude. In a wide range of the parameter space of the models under
consideration the observable hadronic cross sections for top pair production at
the Tevatron and at the LHC are typically reduced by several percent (<~10 %)
compared to the lowest order result. In special regions of the parameter space,
that is in the vicinity of the threshold for the top quark decay t \to b H^+ ;
\tilde t \tilde \chi^0, the radiative corrections are considerably enhanced,
comparable in size to QCD effects.
