  (abridged) The 222000 I-band light curves of variable stars detected by the
OGLE-II survey in the direction of the Galactic Bulge have been fitted and have
also been correlated with the DENIS and 2MASS databases. Results are presented
for 2691 objects with I-band semi-amplitude larger than 0.45 magnitude,
corresponding to classical Mira variables. The Mira period distribution of 6
fields at similar longitude but spanning latitudes from -1.2 to -5.8 degrees
are statistically indistinguisable indicating similar populations with initial
masses of 1.5-2 \msol (corresponding to ages of 1-3 Gyr). A field at similar
longitude at b = -0.05 from Glass et al. (2001) does show a significantly
different period distribution, indicating the presence of a younger population
of 2.5-3 \msol and ages below 1 Gyr. The K-band period-luminosity relation is
presented for the whole sample, and for sub-fields. The zero point depends on
Galactic longitude. Simulations are carried out to show that the observed
dependence of the zero point with $l$, and the number of stars per field are
naturally explained using the model of disk and bulge stars of Binney et al.
(1997), for a viewing angle of 43 +/- 17 degrees. A comparison is made with
similar objects in the Magellanic Clouds, studied in a previous paper. The
slope of the PL-relation in the Bulge and the MCs agree within the errorbars. A
distance to the Galactic Centre between 8.6 and 9.0 kpc is derived (for an
assumed DM of 18.50 to the LMC).
