  The COMPASS experiment at CERN has collected a large sample of events of
inelastic scattering of longitudinally polarised muons off longitudinally
polarised protons in the non-perturbative region (four-momentum transfer
squared $Q^2<1$ GeV$^2$/$c^2$), with a Bjorken scaling variable in the range
$4\times 10^{-5}<x<4\times 10^{-2}$. The data set is two orders of magnitude
larger than the similar sample collected by the SMC experiment. These data
complement our data for polarised deuterons. They allow the accurate
determination of the longitudinal double spin asymmetry $A_1^p$ and of the
spin-dependent structure function $g_1^p$ of the proton in the region of low
$x$ and low $Q^2$. The preliminary results of the analysis of these data yield
non zero and positive asymmetries and of the structure function $g_1^p$. This
is the first time that spin effects are observed at such low $x$.
