  We construct D=4 Type I vacua with massless content remarkably close to that
of the standard model of particle physics. They are tachyon-free
non-supersymmetric models which are obtained starting with a standard D=4, N=1
compact Type IIB orientifold and adding the same number of Dp-branes and
anti-Dp-branes distributed at different points of the underlying orbifold.
Supersymmetry-breaking is felt by the observable world either directly, by
gravity mediation or gauge mediation, depending on the brane configuration. We
construct several simple three generation examples with the gauge group of the
standard model or its left-right symmetric extensions. The models contain a
number of U(1) gauge groups whose anomalies are cancelled by a generalized
Green-Schwarz mechanism. These U(1)'s are broken but may survive as global
symmetries providing for a flavour structure to the models. The value of the
string scale may be lowered down to the intermediate scale (as required in the
gravity mediation case) or down to 1-100 TeV for the non-SUSY models. Thus the
present models are the first semirealistic string vacua realizing the
possibility of a low string scale. The unbalanced force between the pairs of
Dp- and anti-Dp-branes provides for an effect which tends to compactify some of
the extra dimensions but no others. This could provide a new mechanism for
radius stabilization.
