  This study aims to characterise the polarized foreground emission in the
ELAIS-N1 field and to address its possible implications for the extraction of
the cosmological 21-cm signal from the Low-Frequency Array - Epoch of
Reionization (LOFAR-EoR) data. We use the high band antennas of LOFAR to image
this region and RM-synthesis to unravel structures of polarized emission at
high Galactic latitudes. The brightness temperature of the detected Galactic
emission is on average 4 K in polarized intensity and covers the range from -10
to +13rad m^-2 in Faraday depth. The total polarized intensity and polarization
angle show a wide range of morphological features. We have also used the
Westerbork Synthesis Radio Telescope (WSRT) at 350 MHz to image the same
region. The LOFAR and WSRT images show a similar complex morphology, at
comparable brightness levels, but their spatial correlation is very low. The
fractional polarization at 150 MHz, expressed as a percentage of the total
intensity, amounts to 1.5%. There is no indication of diffuse emission in total
intensity in the interferometric data, in line with results at higher
frequencies. The wide frequency range, good angular resolution and good
sensitivity make LOFAR an exquisite instrument for studying Galactic polarized
emission at a resolution of 1-2 rad m^-2 in Faraday depth. The different
polarised patterns observed at 150 MHz and 350 MHz are consistent with
different source distributions along the line of sight wring in a variety of
Faraday thin regions of emission. The presence of polarised foregrounds is a
serious complication for Epoch of Reionization experiments. To avoid the
leakage of polarized emission into total intensity, which can depend on
frequency, we need to calibrate the instrumental polarization across the field
of view to a small fraction of 1%.
