Context : The cosmological 21 cm line promises to be a formidable tool for cosmology , allowing the investigation of the end of the so–called dark ages , when the first galaxies formed .

Aims : Astrophysical foregrounds are expected to be about three orders of magnitude greater than the cosmological signal and therefore represent a serious contamination of the cosmological 21 cm line .
Detailed knowledge of both their intensity and polarization structure on the relevant angular scale of 1–30 arcmin will be essential for extracting the cosmological signal from the data .

Methods : We present the first results from a series of observations conducted with the Westerbork telescope in the 140–160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments .
The polarization data were analysed through the rotation measure synthesis technique .
We computed total intensity and polarization angular power spectra .

Results : For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz , in the low Galactic latitude area known as Fan region , centred at \alpha = 3 ^ { h } 10 ^ { m } , \delta = 65 ^ { \circ } 30 ^ { \prime } .
Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power–law behaviour down to \ell \sim 900 with slope \beta ^ { I } _ { \ell } = -2.2 \pm 0.3 .
The detection of diffuse emission at smaller angular scales is limited by residual point sources .
We measured an rms confusion noise of \sim 3 mJy beam ^ { -1 } .

Diffuse polarized emission was also detected for the first time at this frequency .
The polarized signal shows complex structure both spatially and along the line of sight .
The polarization power spectrum is not affected by residual point sources and is only limited by the thermal noise .
It shows a power–law behaviour down to \ell \sim 2700 with slope \beta ^ { P } _ { \ell } = -1.65 \pm 0.15 .
The rms of polarization fluctuations is 7.2 K on 4 arcmin scales .

Conclusions : The measured total intensity fluctuations are used to estimate the foreground contamination on the cosmological signal .
By extrapolating the spectrum of total intensity emission , we find a contamination of \delta T = \sqrt { \ell ( \ell + 1 ) C ^ { I } _ { \ell } / 2 \pi } \sim 5.7 K on 5 arcmin scales and a corresponding rms value of \sim 18.3 K at the same angular scale .

The level of the polarization power spectrum is \delta T \sim 3.3 K on 5 arcmin scales .
However , the Fan region can not be taken as representative of other sky regions , given its exceptionally bright polarized signal , but is likely to represent an upper limit on the sky brightness at moderate and high Galactic latitude .