  We present deep polarimetric observations at 154 MHz with the Murchison
Widefield Array (MWA), covering 625 deg^2 centered on RA=0 h, Dec=-27 deg. The
sensitivity available in our deep observations allows an in-band,
frequency-dependent analysis of polarized structure for the first time at long
wavelengths. Our analysis suggests that the polarized structures are dominated
by intrinsic emission but may also have a foreground Faraday screen component.
At these wavelengths, the compactness of the MWA baseline distribution provides
excellent snapshot sensitivity to large-scale structure. The observations are
sensitive to diffuse polarized emission at ~54' resolution with a sensitivity
of 5.9 mJy beam^-1 and compact polarized sources at ~2.4' resolution with a
sensitivity of 2.3 mJy beam^-1 for a subset (400 deg^2) of this field. The
sensitivity allows the effect of ionospheric Faraday rotation to be spatially
and temporally measured directly from the diffuse polarized background. Our
observations reveal large-scale structures (~1 deg - 8 deg in extent) in linear
polarization clearly detectable in ~2 minute snapshots, which would remain
undetectable by interferometers with minimum baseline lengths >110 m at 154
MHz. The brightness temperature of these structures is on average 4 K in
polarized intensity, peaking at 11 K. Rotation measure synthesis reveals that
the structures have Faraday depths ranging from -2 rad m^-2 to 10 rad m^-2 with
a large fraction peaking at ~+1 rad m^-2. We estimate a distance of 51+/-20 pc
to the polarized emission based on measurements of the in-field pulsar
J2330-2005. We detect four extragalactic linearly polarized point sources
within the field in our compact source survey. Based on the known polarized
source population at 1.4 GHz and non-detections at 154 MHz, we estimate an
upper limit on the depolarization ratio of 0.08 from 1.4 GHz to 154 MHz.
