  We present the most comprehensive picture ever obtained of the central parts
of the Milky Way probed with RR Lyrae variable stars. This is a collection of
38257 RR Lyr stars detected over 182 square degrees monitored photometrically
by the Optical Gravitational Lensing Experiment (OGLE) in the most central
regions of the Galactic bulge. The sample consists of 16804 variables found and
published by the OGLE collaboration in 2011 and 21453 RR Lyr stars newly
detected in the photometric databases of the fourth phase of the OGLE survey
(OGLE-IV). 93% of the OGLE-IV variables were previously unknown. The total
sample consists of 27258 RRab, 10825 RRc, and 174 RRd stars. We provide OGLE-IV
I- and V-band light curves of the variables along with their basic parameters.
  About 300 RR Lyr stars in our collection are plausible members of 15 globular
clusters. Among others, we found the first pulsating variables that may belong
to the globular cluster Terzan 1 and the first RRd star in the globular cluster
M54. Our survey also covers the center and outskirts of the Sagittarius Dwarf
Spheroidal Galaxy enabling studies of the spatial distribution of the old
stellar population from this galaxy.
  A group of double-mode RR Lyr stars with period ratios around 0.740 form a
stream in the sky that may be a relic of a cluster or a dwarf galaxy tidally
disrupted by the Milky Way. Three of our RR Lyr stars experienced a pulsation
mode switching from double-mode to single fundamental mode or vice versa. We
also present the first known RRd stars with large-amplitude Blazhko effect.
